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05-15-2016, 01:39 AM   #1
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Focal length (crop factor) confusion

First I read this article: Crop Sensor (APS-C) Cameras and Lens Confusion to get some understanding of the problem... But it didn't completely...

Then, checked on my Tamron 90mm F2.8 Macro and my SMC Pentax DA 50mm F1.8 lenses specifications. The Tamron, as far as I can figure, is in fact a lens that is designed to be used on FF sensors (bodies) and can be used on both FF and APS-C meaning that the EXIF data line telling that the declared 90mm's focal length is actually 135mm on 35mm film camera recognizing that my K-S1 is an APS-C sensor camera. So, it's focal length on FF camera is 90mm and on my (APS-C) is 135mm because of the crop factor.
The 50mm DA Pentax lens is, however, designed for APS-C sensors, and not designed to be used on FF sensors thus, I figure, the 50mm focal length is stated to match the usable sensor size and it should not be recalculated... Or should it? The Exif file does recalculate the focal length for 35mm even though the lens can't be used on a FF sensor/camera/body...

Anyway, my question is very simple... If a lens designed to be 90mm on a FF sensor and is used on a APS-C camera, then, it actually acts as a 135mm lens (on APS-C), but the 50mm lens designed for use on APS-C and not supported for use on FF sensors shouldn't be converted to 75mm but is actually 50mm on APS-C? Right?

Or, if unclear... If I use these lenses on my APS-C sensor, is the declared 90mm actually acting as 135mm and is my 50mm lens 50mm or 75mm?



05-15-2016, 01:49 AM - 2 Likes   #2
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QuoteOriginally posted by stein Quote
So, it's focal length on FF camera is 90mm and on my (APS-C) is 135mm because of the crop factor.
No, it's 90mm on both. Forget what format the lens is designed for- focal length never ever changes, as it's an optical property of the lens

QuoteOriginally posted by stein Quote
The 50mm DA Pentax lens is, however, designed for APS-C sensors, and not designed to be used on FF sensors thus, I figure, the 50mm focal length is stated to match the usable sensor size and it should not be recalculated... Or should it?
Again, the focal length is just 50mm per the logic above. (off topic, but the DA 50mm does work just fine on full-frame too)

QuoteOriginally posted by stein Quote
Anyway, my question is very simple... If a lens designed to be 90mm on a FF sensor and is used on a APS-C camera, then, it actually acts as a 135mm lens (on APS-C), but the 50mm lens designed for use on APS-C and not supported for use on FF sensors shouldn't be converted to 75mm but is actually 50mm on APS-C? Right?
Kind of...but "acts as" and "converted" aren't a good way to think about it. The real reason things are the way they are is because the APS-C sensor sees less of the frame than the full frame sensor. Hence, each format has a different field of view with the same lens.

The 50mm has a focal length of 50mm on APS-C and 50mm on full frame.
The 90mm has a focal length of 90mm on APS-C and 90mm on full frame.

The 50mm on APS-C gives the same field of view as a 75mm lens would on full frame.
The 90mm on APS-C gives the same field of view as a 135mm would on full frame.

The 50mm on full frame gives the same field of view as a ~33mm lens would on APS-C.
The 90mm on full frame gives the same field of view as a 60mm lens would on APS-C.


Hope this clears things up. See also:
The Crop Factor Explained: An Animation - Tutorial Videos | PentaxForums.com

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05-15-2016, 01:51 AM - 1 Like   #3
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No. A 90mm lens is always a 90mm lens. What changes is the angle of view. For example, a 50mm lens made for FF (e.g. the FA 50/1.4) produces an image circle which will cover a FF sensor. Put that same lens on an APS-C body and APS-C sensor is only 2/3 the size (width and height) of the FF sensor. You are cropping the image when you use the lens with a smaller sensor. You could say that the rest of the image circle is going to waste, but that is what is meant by the 1.5 crop factor. In rough terms, the 50mm lens on an APS-C sensor is giving you the same angle of view as a 75mm lens on a FF sensor. But it is still a 50mm lens.
05-15-2016, 02:20 AM   #4
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Ok, thanks a lot. One headache less Glad this is really cleared up. So, it IS the field of view, not the focal length after all... My wife is an optician and in the past 30 years, I learned quite a bit about optics and thus the confusion as this conversion of the focal lenght didn't make absolutely no sense to me.

I could ask one more dumb... So... why the heck the EXIF keeps messing with it at all?

OT: I have a "multipractic" Canon SX50 HS (for light travel it's a really good and handy solution) and found a YT video explaining how the 215mm actually gets recalculated to 1200mm equivalent exactly on the example of the field of view on this specific prosumer camera, and interesting enough, the EXIF file coming with the shots made with this one, actually never gives the 35mm equivalent numbers, only the actual focal lenght used for a specific shot.

---------- Post added 05-15-16 at 11:30 ----------

Ok, to prove I understand and your writing wasn't a waste of time...

If I replace my DA 50mm F1.8 with the Pentax-M 50mm F1.7, I'll get the same image, except the excess (difference between the FF and APS-C sensor size / field of view) will not fit on my APS-C sensor, so, I will get a cropped image of what I would get if I had a FF camera shooting from the same position/distance from a certain object...

OK, if I rethink, just for the sake of the argument, let's pretend that the DA 50mm lens wouldn't work with a FF camera...


Last edited by stein; 05-15-2016 at 02:34 AM.
05-15-2016, 02:32 AM   #5
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QuoteOriginally posted by stein Quote
I could ask one more dumb... So... why the heck the EXIF keeps messing with it at all?
In my opinion, it's pointless given the presence of a sensor format field. It just confuses people!

The same goes with retailers listing a lens like the D FA 150-450mm as "225-675 equivalent" on crop.

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05-15-2016, 02:58 AM   #6
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QuoteOriginally posted by PJ1 Quote
A 90mm lens is always a 90mm lens. What changes is the angle of view.
No, the angle of view of a given lens does not change. It will, like the focal length, never change.

In the case of a FF lens projecting an image onto an APS-C sensor, that sensor will just see less of the projection. In other words, a smaller portion of the FF angle of view. The rest of the projected image circle (or angle of view) of the FF lens the APS-C sensor cannot receive because it is outside its borders will disappear somewhere inside the camera's mirror box for want of better explaining it. If you then resize the APS-C image back to the FF boundaries a FF image circle can support then you get a perceived enlargement.

Cheers

Last edited by Schraubstock; 05-15-2016 at 03:01 AM. Reason: typo
05-15-2016, 04:42 AM   #7
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QuoteOriginally posted by Schraubstock Quote
because it is outside its borders will disappear somewhere inside the camera's mirror box for want of better explaining it
Get a K1, overlay the crop screen in the viewfinder, then you can see what your missing.
05-15-2016, 04:44 AM   #8
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QuoteOriginally posted by Schraubstock Quote
No, the angle of view of a given lens does not change. It will, like the focal length, never change.
Well, the angle of view depends on the the lens and recording medium (film, sensor - the viewer). A lens just has a certain maximum angle of view, and a camera might record all of it, or just a part of it.

Anyway, I think the OP question was already answered. And there are more threads about crop factor that have pages and pages of answers.

Simply put, the focal length is a lens property, independent of camera. If you have one camera, all lenses of a certain focal length (for example, 50mm) will give the same angle of view. The photo might look a little different in terms of colours, distortion, sharpness, but the angle of view will be practically the same. Things mostly change if you start using different sized film/sensor. Then you need a lens that paints a larger picture, or you get dark edges. And this is why the same 50mm lens is telephoto on Pentax Q, it is slightly tele on K-5, it is normal on K-1, and it is slightly wide angle on 645Z. Of course, a lens made for 645Z will produce an image circle bigger than a whole Q camera, so it doesn't make much sense to use it. Similarly, a lens made for Q will not cover the whole sensor of the bigger cameras, so it doesn't make sense to use it on those. Its just the difference between painting the same sized image on a stamp vs. a postcard. On a stamp, you only get one eye, on the post card, you get the whole face. Optimal image circle is just enough to cover sensor (and SR, in case of Pentax K mount cameras). It is best to use the lens made for your format. Ignore all equivalence unless you actually use cameras of different formats. That is the only time where a quick "crop factor" calculation can help you, so you can visualize what a lens will look like on the other camera.

05-15-2016, 05:01 AM - 2 Likes   #9
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QuoteOriginally posted by Kerrowdown Quote
then you can see what your missing.
Ha, I removed the lens, had a good look around inside the mirrorbox but couldn't find anything
05-16-2016, 04:22 AM   #10
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QuoteOriginally posted by stein Quote
First I read this article: Crop Sensor (APS-C) Cameras and Lens Confusion to get some understanding of the problem... But it didn't completely...

Then, checked on my Tamron 90mm F2.8 Macro and my SMC Pentax DA 50mm F1.8 lenses specifications. The Tamron, as far as I can figure, is in fact a lens that is designed to be used on FF sensors (bodies) and can be used on both FF and APS-C meaning that the EXIF data line telling that the declared 90mm's focal length is actually 135mm on 35mm film camera recognizing that my K-S1 is an APS-C sensor camera. So, it's focal length on FF camera is 90mm and on my (APS-C) is 135mm because of the crop factor.
The 50mm DA Pentax lens is, however, designed for APS-C sensors, and not designed to be used on FF sensors thus, I figure, the 50mm focal length is stated to match the usable sensor size and it should not be recalculated... Or should it? The Exif file does recalculate the focal length for 35mm even though the lens can't be used on a FF sensor/camera/body...

Anyway, my question is very simple... If a lens designed to be 90mm on a FF sensor and is used on a APS-C camera, then, it actually acts as a 135mm lens (on APS-C), but the 50mm lens designed for use on APS-C and not supported for use on FF sensors shouldn't be converted to 75mm but is actually 50mm on APS-C? Right?

Or, if unclear... If I use these lenses on my APS-C sensor, is the declared 90mm actually acting as 135mm and is my 50mm lens 50mm or 75mm?

You need to go to the raw exif data and review it. Normally there is a focal length , but most viewer programs also show not only the true focal length but the focal length equivalent for 35mm.

This issue has been here a long time but remember focal length is focal length and you'll do fine
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