Can I use a 127.5mm focal length on a crop camera to see what the view would be like using an 85mm full frame lens?

Question

I have a Nikon D7500 which is a DX format camera. I am thinking of getting a Nikon 85mm lens but I only see one that is in FX format. I have a 70-300 lens in DX format and was wondering if I set the focal length on the 70-300 lens to about 127.5 (based on FX format of 85mm x 1.5 = DX format of 127.7mm) will I then see what an 85mm lens with FX format would look like in my D7500? I don't have much room to back up to take pictures at home so I am trying to get an idea of what an 85mm would like like on my camera at home.

Answer 1

No. You should set your zoom to 85mm to see what an 85mm lens looks like on your camera.

The focal length of a lens is a physical property that does not change no matter what size sensor you put behind it.

The distinction between a crop (DX) and full frame (FX) lens is how big an image circle it projects. A crop lens can only cover an APS-C sized sensor, while an full-frame lens projects a bigger circle that can cover both a full-frame (135 format/35mm film) sensor and an APS-C crop sensor (or smaller format).

An 85mm DX lens would look exactly the same as an 85mm FX lens would on your D7500 crop body. On an FX body, the DX lens would vignette (show dark corners) where the image circle of the lens wouldn't cover the sensor.

Crop factor math (1.5x) is for finding which focal lengths equate the field of view on cameras with different-sized sensors. So, an 85mm lens on a DX body would frame approximately the same as a 127.5mm lens would on an FX body. It's something that's useful if you're switching formats to get an approximation of what lenses you might want. Like, say, telling a phone camera only shooter that the "standard" 4.5mm lens in their phone with a 1/3"-format (6x crop factor) sensor has 28mm-equivalence and the "telephoto" lens has 50mm equivalence, to give them a point of reference about lenses and sensor sizes.

Answer 2

No. You should set lens to 85mm. The focal length of the lens do not change magically when you put the lens on crop camera. The marks on the lens are about factory (FF) focal length.

You will have equivalent focal length of 127mm because of the crop factor which mean you will see what FF camera see with lens with such focal length.

Answer 3

You have good answers already. I am just summarizing and posting my cute images :).

1. Focal length does not care about your camera sensor at all.

So, the first answer is, to see how the framing will look in your camera, put your zoom lens at that desired focal length. Put your 70-300 zoom lens on 85mm.

There are differences between a full-frame lens and a DX lens, but it is not about the focal length.

---

Sometimes you need to compare the framing between two cameras, which is as I understand, not your case.

2. The framing is where you need to take into account the sensor size. But I think you have the idea backward.

A. You have a full-frame framing using some lens, in this case, 85mm.

B. On a Crop factor sensor, you simply have a "cropped" framing compared to A.

C. In order to have the same framing as B, back on a Full frame sensor, you need to "zoom in", in this case, something around 128mm. So that is why we say that B (85mm on a crop sensor) is "128mm equivalent".

D. But if your cameras are the opposite, you need to do the opposite. If you have a framing on a crop sensor with 85mm, and you want to see that framing on a full-frame, you need to zoom out. An 85mm framing would be a 56mm framing on a full frame.

Answer 4

You can use 56.7mm focal length on your zoom lens to see what a 85mm lens would look like on your camera when used with a "speed booster", namely a contraption that is sort of the opposite of a tele extender in that it condenses an FX image circle to DX size.

In short, it's a bit the opposite way round: the FX lens captures a wider scene than a 85mm DX lens would, but on a DX sensor, part of the resulting image just goes to waste, meaning that the result is just the same as with a 85mm DX lens. If you want to make use of that extra material, you can use a speed booster (you could not use this with a true DX lens or the smaller image circle would result in black borders around the actual image). The speed booster will let you cover a wider scene and will get the additional captured light (that would otherwise land outside of the sensor region) onto the sensor. That improves the speed compared to an actual 56.7mm lens with the same aperture number.

You can get to 127.5mm using a tele extender but that will result in a loss of speed since then you throw away even more of the image circle originally provided by the 85mm FX lens. Which means that the resulting f-number will be quite lacklustre compared to the size of glass you mounted on your body.

Answer 5

I wish the concept of “crop factor” would disappear, it’s the source of so much confusion. Further, it’s main value, it helps people that have worked with a given format camera for years switch to a different format. For novices and those who never worked a specific format, it has little value.

If you mount an 85mm lens on a full frame camera the angle of view realized is 16° height, 24° length and 43° diagonal. Of these values the diagonal angle of view is the one most likely published. I think its the least valuable but keep in mind, TV sets are sold based on their diagonal measure. Why, its sounds better, it’s the biggest measurement.

If you mount 57mm lens on a DX, this lash-up will deliver approximately the same angles of view. Why 57mm?. The crop factor (magnification factor) is 1.5 so 85 ÷ 1.5 = 57 (rounded).

Bottom line – you can’t set your 70 – 300 to 57mm so your question is moot.

About angle of view – Every format has a “normal” focal length. This occurs when you mount a lens with a focal length approximately equal to the diagonal measure of the format. For the FX that’s about 43mm, for the DX that’s about 30mm. Such lash-ups deliver a horizontal angle of view of about 45° and that’s considered “normal”.

A shorter lens delivers a wide-angle, a longer angle of view is telephoto. Also, the crop factor 1.5 means 1/1.5 = 0.66 = 66% in other words, the DX format is smaller than an FX it is 66% of the size of a FX the apparent view through the viewfinder appears enlarged 1.5X as compared to the view through the viewfinder of an FX, both with the same focal length lens mounted.