Cameras Ranked
Translating a Lens Spec Between Sensor Formats
After this page you will be able to work out which focal length gives the framing you want on your own sensor, which lens to buy for a look quoted in full-frame terms, how wide that is in degrees, and whether an f-number quoted for a different format is a fair comparison for the thing you actually care about.
Every sensor dimension below is copied from a manufacturer specification page and cited with the date it was read. Where a number is computed rather than published, the page shows the computation.
The short answer
A 35mm lens on an APS-C body frames like a 53mm or a 56mm lens on full frame, depending on whose APS-C sensor you have. That is a normal lens, not a wide one. On Four Thirds it frames like 70mm. The lens has not changed; a smaller sensor behind it records a smaller part of the circle of light it projects. Going the other way you divide: the 50mm full-frame look is about 33mm on a 1.53x body, 31mm on a 1.62x body, and 25mm on Four Thirds.
On aperture you are asking one of two questions, and the same number answers only one. Will your camera need more light? No: f/1.8 is f/1.8 on every format, and your meter reading, shutter speed and ISO do not change. Will the background blur like an f/1.8 shot on full frame? No, and about f/2.75 is the fair comparison on a 1.53x body.
Where the crop factor comes from
Crop factor is the diagonal of a 36 x 24 mm full-frame sensor divided by the diagonal of yours. Canon's EOS R5 manual states that images are captured at approximately 36.0 x 24.0 mm in full-frame shooting. That is one body's published figure, used here as the reference rather than as a claim about every full-frame camera. By Pythagoras its diagonal is the square root of 1872, or 43.27 mm. Every crop factor below is 43.27 divided by the sensor's own diagonal.
| Body cited | Sensor W x H (mm, published) | Diagonal (mm, calculated) | Crop factor (calculated) | Maker's own rounded figure |
|---|---|---|---|---|
| Canon EOS R5 (full frame) | 36.0 x 24.0 (approx.) | 43.27 | 1.00 | reference format |
| Sony a7 IV (35 mm full frame) | 35.9 x 23.9 | 43.13 | 1.00 | quotes approx. 1.5x for its APS-C crop mode |
| Fujifilm X-T5 (APS-C) | 23.5 x 15.7 | 28.26 | 1.53 | not stated on the spec page |
| Canon EOS R7 (APS-C) | 22.3 x 14.8 (approx.) | 26.76 | 1.62 | Canon states approx. 1.6 |
| OM System OM-1 Mark II (Four Thirds) | 17.3 x 13.0 (4:3) | 21.64 | 2.00 | not stated in the sensor block; the image-stabiliser footnotes give 40mm as 80mm equivalent and 150mm as 300mm equivalent, both exactly 2x |
Two things in that table. First, the familiar figures are the computed ones rounded off: 1.53 gets quoted as 1.5, and Canon states its own 1.62 as approximately 1.6. The arithmetic is real; the tidy number is a rounding. Four Thirds is the one that lands genuinely round, since 21.64 doubled is 43.28 against the 43.27 reference. Second, APS-C is not one size. Canon's measures 22.3 x 14.8 mm and Fujifilm's 23.5 x 15.7 mm, so when a lens is advertised as a 50mm equivalent on APS-C, it is fair to ask whose.
Converting a focal length, and reading it in degrees
Equivalent focal length equals actual focal length times crop factor, and tells you which full-frame focal length would frame the same scene from the same standing position. The lens has not become a different lens; the smaller sensor crops into the middle of what it projects. An equivalent focal length only helps if you already know how wide a 53mm lens is, so the table below gives the angle of view too. Diagonal angle of view is 2 x arctan(d / 2f), for d the sensor diagonal and f the focal length: the geometry for a lens focused at infinity, ignoring distortion.
| Lens focal length | Full frame (43.27 mm diag.) | 1.53x Fujifilm APS-C | 1.62x Canon APS-C | 2.00x Four Thirds |
|---|---|---|---|---|
| 18 mm | 18 mm / 100.5 deg | 27.5 mm / 76.3 deg | 29.2 mm / 73.3 deg | 36.0 mm / 62.0 deg |
| 23 mm | 23 mm / 86.5 deg | 35.2 mm / 63.1 deg | 37.3 mm / 60.4 deg | 46.0 mm / 50.4 deg |
| 35 mm | 35 mm / 63.4 deg | 53.6 mm / 44.0 deg | 56.7 mm / 41.8 deg | 70.0 mm / 34.4 deg |
| 50 mm | 50 mm / 46.8 deg | 76.5 mm / 31.6 deg | 81.0 mm / 30.0 deg | 100.0 mm / 24.4 deg |
| 300 mm | 300 mm / 8.2 deg | 459.0 mm / 5.4 deg | 486.0 mm / 5.1 deg | 600.0 mm / 4.1 deg |
The division matters more than the multiplication, because people arrive asking not "I own a 35mm, what is it like" but "I want the 50mm look, what do I buy." Divide the focal length you want by your crop factor. The 50mm look is 32.7mm on a 1.53x body, 30.9mm on a 1.62x body and 25mm on Four Thirds. The 85mm portrait look is 55.6mm, 52.5mm and 42.5mm. Round to a length your maker actually publishes, then check that lens's stated equivalent against your own crop factor.
One caveat on the Four Thirds column. A diagonal ratio preserves framing only when both formats share an aspect ratio, and Four Thirds is 4:3 against a 3:2 reference. Dividing widths gives 36 / 17.3 = 2.08 and heights 24 / 13.0 = 1.85, so a 25mm lens on Four Thirds covers roughly what a 52mm covers across a full-frame frame and what a 46mm covers down it, against the 50mm the diagonal predicts. Treat 2.00x as exact for the diagonal, which is the figure the makers quote, and as an approximation for framing. The APS-C columns sit within half a percent of 3:2 and are unaffected.
Both halves of the table check out against published figures. Fujifilm lists the XF23mmF1.4 R LM WR focal length as f=23mm with 35mm in parentheses, and 23 x 1.53 is 35.2. Sony lists the E 18-135mm at an equivalent 27 to 202.5 mm with an angle of view of 76 to 12 degrees, footnoted as assuming an APS-C sized sensor; the formula on a 28.26 mm diagonal gives 76.3 degrees at 18mm and 11.9 at 135mm. Fujifilm publishes 63.4 degrees for the XF23mmF1.4 where the formula gives 63.1. And 35mm on full frame computes to 63.4 degrees, the same figure Fujifilm publishes for a 23mm lens on APS-C: the equivalence idea turning up in two makers' own numbers.
Sony separately calls its crop setting an angle of view approximately 1.5 times the focal length on the lens. That phrasing is loose and worth reading past: an angle is not a focal length, and the angle of view narrows rather than grows. The 1.5 is right. Read it as a focal-length multiplier.
The aperture question, which is really two questions
Decide which question you are asking before you accept a number, because the crop factor belongs in one answer and not the other.
Question one: will my camera need more light?
No. An f-number is the lens's focal length divided by the diameter of its entrance pupil. Both quantities are properties of the lens; nothing in the definition refers to a sensor. So f/1.8 delivers the same illumination per square millimeter at the image plane whether the sensor behind it is 17.3 mm or 36 mm wide, and your meter reading, shutter speed and ISO need no adjustment for format.
Be clear about what is sourced there. The definition is quoted from MicroscopyU, Nikon Instruments' microscopy education site, and it is written for microscope objective lenses. It is the ratio photographic f-numbers are built on, but this page did not manage to open a photographic standards document stating it, and would rather say so than dress the source up. The step from that definition to "no exposure change across formats" is inferred, not quoted.
Question two: will it look the same?
No, and here the crop factor applies to both numbers at once. Multiply the focal length and the f-number by the same crop factor. On a 1.53x Fujifilm body a 35mm f/1.8 compares to roughly a 53.6mm f/2.75 on full frame. On Four Thirds a 25mm f/1.8 compares to a 50mm f/3.6.
The reason is one line you can check. Entrance pupil diameter is focal length divided by f-number: for the 35mm f/1.8, 35 divided by 1.8 is 19.44 mm. Multiply both by 1.53 and you get 53.55mm at f/2.754, and 53.55 divided by 2.754 is 19.44 mm again. The factor cancels, so the diameter is identical rather than merely close. Any small difference you see in a worked example is the rounding used in the tables above, not physics.
One more step follows, and it deserves a label rather than a slide past. Same entrance pupil plus same angle of view is the standard account of why depth of field and total light gathered match across formats. That step is inferred, and it is an approximation rather than an identity: it assumes the same shooting distance and the same viewing size, and it holds least well at close focus. This page cites no source for the depth-of-field relationship itself. So "an f/1.8 crop lens is really f/2.75" is half right. It compares blur and total light collected. It says nothing about exposure, and nobody should set a camera by it.
What happens in each direction
A full-frame lens on a crop body projects a larger circle than the sensor needs. You lose the outer part, and the field of view narrows by the crop factor. Canon lists the APS-C EOS R7 as supporting both the RF and RF-S lens groups on one RF mount, and EF and EF-S lenses through the Mount Adapter EF-EOS R.
A crop lens on a full-frame body is where the makers differ. Canon is prescriptive: with an RF-S or EF-S lens on a full-frame EOS R5 the 1.6x crop is set automatically, Canon states no other option is available, and areas outside the crop are not recorded in RAW, so there is no hidden full-frame file to recover. Nikon does the same, its DX and FX Formats page stating that a DX lens on an FX camera automatically engages the built-in DX crop and records only the center section of the sensor. Sony is permissive: the a7 IV's APS-C S35 setting offers On, Auto and Off, where Off always captures the full 35mm frame, and Sony warns that leaving it off with an APS-C lens may darken the edges.
Whether a lens physically mounts, and whether autofocus and aperture control survive, is a separate question from framing, covered at the lens mount guide. Which movie modes survive a crop setting, and which card sustains them, is covered at the memory card guide.
What the crop costs in pixels
Work this out on your own camera rather than trusting a general figure. The cropped pixel count is roughly the full count divided by the crop factor squared, since you keep a smaller rectangle in both dimensions. That is inferred from the geometry, and it is an estimate. Canon lists a Large full-frame EOS R5 file at approximately 44.8 megapixels (8192 x 5464). Divide 44.8 by 1.6 squared, 2.56, and you predict about 17.5 megapixels; Canon publishes approximately 17.3 megapixels (5088 x 3392) for the 1.6x crop. Close but not exact, so treat the division as a planning number rather than a specification.
What to look up on your own body and lens
- On your body's specification page, find the image sensor line and read the dimensions in millimeters. Canon's product manual labels the field "Screen size". Sony writes the dimensions inside the sensor type line. OM System labels it "Aspect ratio & area". Fujifilm puts them at the front of the sensor line. Do not accept the words "APS-C" as the answer; get the two numbers.
- Compute your own crop factor rather than looking it up: 43.27 divided by the square root of (width squared plus height squared). That is the only figure this page treats as authoritative for your camera. While the numbers are out, divide width by height too. If that is not close to 1.5, the aspect-ratio caveat above applies to you and your equivalent focal lengths are approximate.
- On the lens page, find the coverage designation. Canon separates RF from RF-S, and the EOS R7 specifications list RF and RF-S as two lens groups on one mount. Nikon marks only the crop lenses: its DX and FX Formats page states that the DX designation appears in the lens name, so on a Nikon lens the absence of DX is your answer. This page cites no naming convention for Sony or Fujifilm, so there use the maker's own fields instead. Sony footnotes its equivalent focal length and angle of view as assuming an APS-C sensor, and a lens that already covers 36 x 24 mm has no equivalent figure to publish at all.
- A shared mount name does not guarantee the lens will attach, and this is the step to be careful on. The two cases this page can source both work: Canon lists RF and RF-S as groups on the one RF mount, and Nikon documents a DX lens on an FX body engaging the DX crop. Do not generalise from those to other systems, to adapters, or to older SLR mounts. Whether a crop-format lens fits a particular full-frame body is a per-body fact, documented in that body's own manual, which is where to read it before you buy or borrow. If a lens does not seat easily, stop and check rather than forcing it.
- If the lens page publishes a "35mm format equivalent" figure, check it against your own crop factor before relying on it. That figure was computed for the maker's own sensor, which may not be the size of yours.
- If the lens page publishes an angle of view in degrees, prefer it to the equivalent focal length. Fujifilm gives 63.4 degrees for the XF23mmF1.4; Sony gives 76 to 12 degrees for the E 18-135mm. A degree figure already describes the coverage you get on the format it was quoted for, so there is nothing to convert.
What this page does not tell you
It offers no opinion on whether a format is better. Larger and smaller sensors each trade something away, and this page has no view on the trade. It also excludes third-party adapters and lenses. Camera makers generally do not document how another company's optics or electronics behave on their bodies, and what they do say can change with firmware. If your lens or adapter did not come from your camera's manufacturer, the spec fields above are still the right ones to read, but the answer has to come from whoever made the part.
Check it on your own equipment
These take you to the manufacturers and standards bodies the page draws on, so you can look up your exact model.
- Manufacturer specification
Canon EOS R5 manual: Still Photo Cropping/Aspect Ratio (opens in a new tab)Canon's own page stating the approximately 36.0 x 24.0 mm full-frame capture size, the approximately 1.6 crop factor, the automatic crop forced by RF-S and EF-S lenses, and the statement that areas outside the crop are not recorded in RAW. Change the C003 code in the URL for a different EOS body. — cam.start.canon - Official support page
Nikon: The DX and FX Formats (opens in a new tab)Nikon's statement that the DX designation appears in the lens name, which is why a Nikon full-frame lens carries no FX marking and the absence of DX is the answer, plus Nikon's description of the automatic DX crop engaged when a DX lens is mounted on an FX body. — www.nikonusa.com - Official support page
Sony a7 IV Help Guide: APS-C S35 (Super 35mm) Shooting (opens in a new tab)The menu path to Sony's crop setting, the On / Auto / Off behaviours, the approximately 1.5x figure in Sony's own loose wording, and Sony's warning about darkened edges when an APS-C lens is used with the setting off. — helpguide.sony.net - Manufacturer specification
OM System OM-1 Mark II specification sheet (PDF) (opens in a new tab)A downloadable sheet where the sensor size appears under a field labeled 'Aspect ratio & area' reading 4:3 / 17.3 x 13.0 mm — both the 4:3 figure that drives the aspect-ratio caveat on this page and an example of how differently makers label the same number. The image-stabiliser footnotes give 40mm as 80mm equivalent. — my.omsystem.com
What this page does not tell you
- Every sensor dimension is copied from a manufacturer page cited above; every diagonal, crop factor, equivalent focal length and angle of view is arithmetic performed on those published dimensions and is labeled inferred.
- Manufacturers publish rounded or approximate dimensions. Canon writes 'approx. 36.0 x 24.0 mm' and 'approx. 22.3 x 14.8 mm'. Crop factors computed from rounded inputs carry that rounding, which is why the page shows 1.62 where Canon says approximately 1.6, and why Fujifilm's published 63.4 degree angle of view sits 0.3 degrees from the 63.1 the rounded dimensions produce.
- The 2.00x Four Thirds figure is exact for the diagonal and approximate for framing, because Four Thirds is 4:3 and the full-frame reference is 3:2. The true equivalent runs from about 1.85x vertically to about 2.08x horizontally. Any reader whose sensor is not close to 3:2 should treat the equivalent focal lengths in the conversion table as approximations.
- The angle-of-view figures assume a rectilinear lens focused at infinity and ignore distortion; they drift at close focusing distances. They are computed, not published, except where the page explicitly quotes Sony's 76 to 12 degrees or Fujifilm's 63.4 degrees.
- The f-number definition this page cites is from MicroscopyU, Nikon Instruments' microscopy education site, and is written for microscope objective lenses. The page did not succeed in opening a photographic standards document stating the same ratio; ISO 517 is the relevant standard and iso.org returned HTTP 403 to this page's requests, so it is not cited here.
- The step from 'same entrance pupil diameter and same angle of view' to 'same depth of field and same total light gathered' is inferred and approximate. It assumes matched shooting distance and matched viewing enlargement, and holds least well at close focus. No source is cited for the depth-of-field relationship itself.
- The pixel-count division by crop factor squared is an approximation, not a specification. Against Canon's own EOS R5 figures it predicts 17.5 megapixels where Canon publishes approximately 17.3, so use it for planning only.
- On mount compatibility, this page claims only what it opened: Canon listing RF and RF-S as two lens groups on one RF mount, Canon's EOS R5 behavior with RF-S and EF-S lenses, and Nikon's DX-on-FX behavior. It makes no claim about any SLR mount pairing, about adapters, or about any body it does not cite. Read your own body's lens-compatibility documentation before mounting a lens from a different coverage class.
- This page cites no naming convention for Sony's or Fujifilm's lens designations. It cannot tell a Sony or Fujifilm owner what a prefix means, and instead directs them to the maker's own equivalent-focal-length and angle-of-view fields, which it did open.
- Equivalent focal length describes framing from the same standing position. It is not a claim about perspective, compression, optical quality or rendering, none of which this page addresses.
- Only five bodies and two lenses are cited. They are used as sources for their formats' dimensions, not as recommendations, and no claim here is made about any other model without checking its own specification page.
- Third-party lenses, teleconverters and mount adapters are out of scope.
Sources
Manufacturers revise specifications. Every source below is dated — open it before you buy.
- The Sony a7 IV image sensor is 35 mm full frame measuring 35.9 mm x 23.9 mm, CMOS type, with approximately 33,000,000 effective pixels and a Sony E-mount.
- The Fujifilm X-T5 sensor is 23.5 mm x 15.7 mm (APS-C), an X-Trans CMOS 5 HR with a primary color filter, 40.2 million effective pixels, using the FUJIFILM X mount.
- The OM System OM-1 Mark II uses a Micro Four Thirds lens mount and a 4/3 inch Stacked BSI Live MOS sensor whose aspect ratio and area are given as 4:3 / 17.3 x 13.0 mm, with 20.4 megapixels effective.
- The OM System OM-1 Mark II specification sheet states a 2x relationship between marked and 35mm-equivalent focal length in its image-stabiliser footnotes, giving 40mm as 80mm equivalent and 150mm as 300mm equivalent.
- Canon's EOS R5 manual states that images are captured at a sensor size of approximately 36.0 x 24.0 mm when shooting full frame.
- Canon describes its crop setting as isolating and magnifying the center of the image by approximately 1.6, which it labels APS-C size.
- On a full-frame Canon EOS R5, an RF-S or EF-S lens causes the approximately 1.6x crop to be set automatically, and Canon states no other option is available.
- Canon states that areas outside the cropped area are not recorded in RAW shooting when the 1.6x crop is set or when an RF-S or EF-S lens is fitted.
- Canon rates the EOS R5 at a maximum of approximately 45.0 megapixels effective on a CMOS sensor with a screen size of approximately 36.0 x 24.0 mm.
- Canon lists a Large full-frame EOS R5 file as approximately 44.8 megapixels at 8192 x 5464, and the corresponding 1.6x crop file as approximately 17.3 megapixels at 5088 x 3392.
- The Canon EOS R7 has an APS-C CMOS sensor of approximately 22.3 x 14.8 mm with a maximum of approximately 32.5 megapixels effective.
- The APS-C Canon EOS R7 uses the Canon RF mount and supports both the RF and RF-S lens groups, with EF and EF-S lenses usable via the Mount Adapter EF-EOS R.
- The Sony a7 IV has an APS-C S35 Shooting setting with On, Auto and Off; On gives an angle of view Sony describes as approximately 1.5 times the focal length indicated on the lens, and Off always captures the full 35mm frame.
- Sony warns that attaching an APS-C-compatible lens with the APS-C S35 setting disabled may cause the edges of the image to become darker.
- Sony publishes the E 18-135mm F3.5-5.6 OSS as having an equivalent 35 mm-format focal length of 27 to 202.5 mm and an equivalent 35 mm-format angle of view of 76 to 12 degrees, footnoted as based on cameras with an APS-C sized image sensor.
- Fujifilm publishes the XF23mmF1.4 R LM WR focal length as f=23mm with 35mm in parentheses, a maximum aperture of F1.4, and an angle of view of 63.4 degrees.
- The f-number is defined as the ratio of the focal length of the lens to the diameter of the entrance pupil. The source stating this is Nikon Instruments' MicroscopyU, a microscopy education site, and the definition there is written for microscope objective lenses.
- Nikon states that the DX designation appears in the lens name, so Nikon full-frame (FX) lenses are not themselves marked FX and the absence of a DX marking is what identifies full-frame coverage.
- Nikon states that with a DX lens mounted on a full-frame FX-format camera, the camera automatically engages its built-in DX crop mode and records an image only from the center section of the sensor.
- The diagonal of a 36.0 x 24.0 mm full-frame sensor is 43.27 mm, and this is the reference figure from which every crop factor on this page is calculated.
- The diagonal of the Sony a7 IV's 35.9 x 23.9 mm sensor is 43.13 mm, giving a crop factor of 1.00 against the 43.27 mm reference.
- The diagonal of the Fujifilm X-T5's 23.5 x 15.7 mm sensor is 28.26 mm, giving a computed crop factor of 1.53.
- The diagonal of the Canon EOS R7's 22.3 x 14.8 mm sensor is 26.76 mm, giving a computed crop factor of 1.62, consistent with Canon's own rounded figure of approximately 1.6.
- The diagonal of the OM-1 Mark II's 17.3 x 13.0 mm sensor is 21.64 mm, giving a computed crop factor of 2.00; 21.64 doubled is 43.28 against the 43.27 reference, so the round figure survives rounding rather than being produced by it.
- The familiar 1.5x and 1.6x figures are the computed crop factors rounded: 1.53 for Fujifilm is quoted as 1.5, and Canon's computed 1.62 is stated by Canon as approximately 1.6.
- APS-C is not a single sensor size: Canon's is 22.3 x 14.8 mm giving 1.62, while Fujifilm's is 23.5 x 15.7 mm giving 1.53, so an equivalent focal length quoted for one does not transfer to the other.
- Equivalent focal length equals actual focal length multiplied by crop factor; the lens's focal length itself does not change, only the angle of view captured by the smaller sensor.
- Sony's phrasing 'the angle of view becomes approximately 1.5 times the focal length indicated on the lens' is physically loose, since an angle is not a focal length and the captured angle of view narrows rather than growing; the 1.5 figure is correct read as a focal-length multiplier.
- A 35mm lens converts to approximately 53.6mm on a 1.53x body, 56.7mm on a 1.62x body and 70.0mm on a 2.00x body; a 50mm converts to 76.5mm, 81.0mm and 100.0mm; a 300mm converts to 459.0mm, 486.0mm and 600.0mm.
- To find the lens that gives a look quoted in full-frame terms, divide by your crop factor: the 50mm full-frame look is 32.7mm on a 1.53x body, 30.9mm on a 1.62x body and 25mm on Four Thirds; the 85mm look is 55.6mm, 52.5mm and 42.5mm.
- Because Four Thirds is 4:3 and the full-frame reference is 3:2, the 2.00x diagonal crop factor does not preserve framing exactly: dividing widths gives 36 / 17.3 = 2.08 and heights 24 / 13.0 = 1.85, so a 25mm lens on Four Thirds covers roughly what a 52mm covers horizontally and a 46mm vertically on full frame, against the 50mm the diagonal predicts.
- Fujifilm's published 35mm equivalent of 35mm for the 23mm XF23mmF1.4 is consistent with the 1.53 crop factor computed from the X-T5 sensor dimensions, since 23 multiplied by 1.53 is 35.2.
- Sony's published 27 to 202.5 mm equivalent for the 18-135mm lens is exactly the 1.5x multiplication, since 18 times 1.5 is 27 and 135 times 1.5 is 202.5.
- The diagonal angle of view of a lens focused at infinity is 2 x arctan(d / 2f), where d is the sensor diagonal and f the focal length; on the diagonals computed above this gives, for example, 100.5 degrees for 18mm on full frame, 76.3 degrees for 18mm on a 1.53x body, 63.4 degrees for 35mm on full frame and 24.4 degrees for 50mm on Four Thirds.
- The angle-of-view formula reproduces Sony's published figures: on a 28.26 mm APS-C diagonal it gives 76.3 degrees at 18mm and 11.9 degrees at 135mm, against Sony's published 76 to 12 degrees for the E 18-135mm.
- The angle-of-view formula gives 63.1 degrees for the XF23mmF1.4 on the X-T5's computed 28.26 mm diagonal against Fujifilm's published 63.4 degrees; the 0.3 degree gap implies Fujifilm computed from an image diagonal nearer 28.4 mm than the rounded 23.5 x 15.7 dimensions yield.
- A 35mm lens on full frame computes to a 63.4 degree diagonal angle of view, the same figure Fujifilm publishes for the 23mm XF23mmF1.4 on APS-C, which is the focal-length equivalence appearing in two makers' own published numbers.
- The f-number that sets exposure does not change with sensor format, because f-number is focal length divided by entrance pupil diameter and both are properties of the lens alone.
- Multiplying both the focal length and the f-number by the same crop factor leaves the entrance pupil diameter exactly unchanged, because the factor cancels: 35 divided by 1.8 is 19.44 mm, and 53.55 divided by 2.754 is 19.44 mm.
- The same entrance pupil diameter combined with the same angle of view is the standard account of why depth of field and total light gathered match across formats, but this is an approximation that assumes the same shooting distance and viewing enlargement and degrades at close focus, and this page cites no source for the depth-of-field relationship itself.
- On a 1.53x body a 35mm f/1.8 compares to roughly a 53.6mm f/2.75 on full frame, and on Four Thirds a 25mm f/1.8 compares to a 50mm f/3.6.
- The pixel count of a cropped area is approximately the full pixel count divided by the crop factor squared, an estimate that lands close to but not exactly on the manufacturer's published figure.
- A full-frame lens mounted on a matching crop body projects a larger image circle than the sensor requires, and the field of view narrows by the crop factor.
- A shared mount name does not by itself establish that a lens of a different coverage class will attach to a given body; the two cross-class pairings this page can source are Canon's RF/RF-S on the RF mount and Nikon's DX on FX, and whether any other crop-format lens fits a particular full-frame body is documented in that body's own manual.
- If a lens does not seat easily on a body, stop and check the body's documentation rather than forcing it.