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11-20-2013, 12:14 PM   #1
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film lens aperture on crop sensor

Hi,

First of all please excuse my noob thread.
I have the Pentax M 50mm f1.7 manual focus lens and the K-5ii. I know about 1.5 crop factor of the focal distance.
But how about the aperture? I know the physical aperture does not change, the lens is still a f1.7 and the amount of light that enters is the same regardless of the sensor size.
My question is will I get the same exposure as of a 50mm f1.7 specifically designed for crop sensor? I'm talking here strictly about the low light situations, not the depth of field.
My canonist friend has a plasticfantastic 50mm f 1.8 (crop sensor, crop lens) and I think in low light situations he has an advantage, though I'm not sure, I haven't done a comparison test.
My logic is that because of the FF lense's larger image circle, on a crop sensor some of the light will be "lost", passing by the sensor. Is that correct? So even if the lens is still f1.7 the effect will be that of a slower lens right?

So simply forward my F 1.7 will be like a F..(?)... on my K5.

Thank you for your patience and sorry for my bad english.
Raul

11-20-2013, 12:26 PM   #2
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QuoteOriginally posted by raultimis Quote
Hi,

First of all please excuse my noob thread.
I have the Pentax M 50mm f1.7 manual focus lens and the K-5ii. I know about 1.5 crop factor of the focal distance.
But how about the aperture? I know the physical aperture does not change, the lens is still a f1.7 and the amount of light that enters is the same regardless of the sensor size.
My question is will I get the same exposure as of a 50mm f1.7 specifically designed for crop sensor? I'm talking here strictly about the low light situations, not the depth of field.
My canonist friend has a plasticfantastic 50mm f 1.8 (crop sensor, crop lens) and I think in low light situations he has an advantage, though I'm not sure, I haven't done a comparison test.
My logic is that because of the FF lense's larger image circle, on a crop sensor some of the light will be "lost", passing by the sensor. Is that correct? So even if the lens is still f1.7 the effect will be that of a slower lens right?

So simply forward my F 1.7 will be like a F..(?)... on my K5.

Thank you for your patience and sorry for my bad english.
Raul
You get the same exposure, same amount of light, same DOF, same everything as you would on FF. APS-C just sees less of the frame.

Adam
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11-20-2013, 12:28 PM   #3
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Edit: TL; DR see Adam's post above

The amount of light entering the mirror box and falling on the sensor doesn't change (f/1.7 = f/1.7 = f/1.7). You're just using a smaller part of that total covered area. Imagine shooting the lens on film. Does the centre of the film have a different exposure? (disregarding vignetting of course)

The Canon lens might have a bit better coatings permitting a bit more light through, or it might be the Pentax way of the camera refusing to blow highlights at all cost, or it might be incorrect metering because it's a manual lens.
11-20-2013, 01:47 PM   #4
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Aperture is the ratio of the focal length of the lens to the diameter of the opening.

Your 50mm lens has a max aperture that's about 29.4mm wide, your friend's is about 27.8. It's a 1/4 stop difference in your favor -- i.e., barely noticeable.

If you took both cameras, set them to manual, set the aperture to 1.7 and 1.8, the shutter speed to the same speed, and the ISO rating to the same rating, and both cameras had things like D-Range settings, highlight/shadow recovery, whatever canon calls their version, turned off, and they were sitting right next to each other, the pentax should get an image that is very slightly brighter than the canon (unless, as Giklab noted, coatings play a factor). But it still may not look that way on the camera. His screen could be brighter than yours. Export them both in RAW into lightroom and compare them there, maybe, if you want to go to the trouble.

He does have a couple of at least perceptual advantages though… autofocus means he's not squinting to get the focus right; and more importantly, autofocus is only going to work if it's bright enough for it to focus on something, so he won't be taking pictures of anything that's too dark for AF to work, unless he's manually focusing or snapping a flashlight or something at it to help the focus and turning it off before he takes the picture.

11-20-2013, 01:56 PM   #5
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QuoteOriginally posted by Adam Quote
same DOF,
Not quite, but that is the subject of numerous other threads.


Steve
11-20-2013, 02:49 PM   #6
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QuoteOriginally posted by stevebrot Quote
Not quite, but that is the subject of numerous other threads.


Steve
You get the same DOF with the same lens (think of it as just cropping the FF image). You get a different DOF at equivalent fields of view (i.e. different focal lengths), however. And yes, there are plenty of topics on that already...

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11-20-2013, 10:40 PM   #7
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QuoteOriginally posted by Adam Quote
You get the same DOF with the same lens (think of it as just cropping the FF image). You get a different DOF at equivalent fields of view (i.e. different focal lengths), however. And yes, there are plenty of topics on that already...
Er, sorry Adam - but the smaller the sensor, the smaller the DOF (for any given lens). The reason is that the smaller sensor sees a greater degree of out of focus blur. If you want confirmation that it works like this, have a play with this DOF calculator:

A Flexible Depth of Field Calculator

If you had a high-megapixel image and did some cropping, you'd see the same effect (as long as you viewed the cropped image at the same size as the uncropped image).
11-20-2013, 10:58 PM   #8
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QuoteOriginally posted by m42man Quote
Er, sorry Adam - but the smaller the sensor, the smaller the DOF (for any given lens). The reason is that the smaller sensor sees a greater degree of out of focus blur. If you want confirmation that it works like this, have a play with this DOF calculator:

A Flexible Depth of Field Calculator

If you had a high-megapixel image and did some cropping, you'd see the same effect (as long as you viewed the cropped image at the same size as the uncropped image).
Based on what I know:

QuoteQuote:
  1. For an equivalent field of view, a small-sensor camera has MORE depth of field than a full-frame camera would have - when the focus distance is significantly less then the hyperfocal distance.
  2. Using the same lens on a small-sensor camera and a full-frame camera, the small-sensor image has LESS depth of field than the full-frame image would have (but they would be different images since the field of view would be different)
  3. If you use the same lens on a small-sensor camera and a full-frame camera and crop the full-frame image to give the same view as the digital image, the depth of field is IDENTICAL



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11-20-2013, 11:46 PM   #9
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"If you use the same lens on a small-sensor camera and a full-frame camera and crop the full-frame image to give the same view as the digital image, the depth of field is IDENTICAL"

I think that's just saying that a cropped image from a full-frame sensor will be identical to the full image from the smaller sensor (including DOF) - which is obviously exactly as you'd expect. It shouldn't be interpreted as "the DOF of the uncropped full-frame image will be the same as the DOF of the cropped image" (because it isn't).
11-21-2013, 12:25 AM   #10
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QuoteOriginally posted by Adam Quote
And yes, there are plenty of topics on that already...
Though you are still not quite correct (wrong is such a strong word). It is a complicated subject, but easy enough to demonstrate. Cropping by itself will retain the same DOF though the final image dimensions at the sensor will be smaller (the crop). Normalizing the FOV (same subject, different distances) or final image size (same subject, same distance, greater/less enlargement) will result in different DOF. I don't remember the exact math, but it was once put to me succinctly (on this forum) that ultimately the only factors are absolute aperture, final magnification, and viewing distance.


Steve

(...sorry to derail this thread...)
11-21-2013, 01:37 AM   #11
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QuoteOriginally posted by m42man Quote
I think that's just saying that a cropped image from a full-frame sensor will be identical to the full image from the smaller sensor (including DOF) - which is obviously exactly as you'd expect. It shouldn't be interpreted as "the DOF of the uncropped full-frame image will be the same as the DOF of the cropped image" (because it isn't).
Exactly!

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11-21-2013, 01:38 AM   #12
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QuoteOriginally posted by m42man Quote
Er, sorry Adam - but the smaller the sensor, the smaller the DOF (for any given lens). The reason is that the smaller sensor sees a greater degree of out of focus blur. If you want confirmation that it works like this, have a play with this DOF calculator:

A Flexible Depth of Field Calculator

If you had a high-megapixel image and did some cropping, you'd see the same effect (as long as you viewed the cropped image at the same size as the uncropped image).
Read carefully at the first point of Adam's quote:

"1. For an equivalent field of view,"

That mean you use significantly wider lens on small sensor to achieve the same FOV on the large sensor (which uses longer lens), therefore smaller sensor display more DOF.
11-21-2013, 04:22 AM   #13
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QuoteOriginally posted by m42man Quote
"If you use the same lens on a small-sensor camera and a full-frame camera and crop the full-frame image to give the same view as the digital image, the depth of field is IDENTICAL"

I think that's just saying that a cropped image from a full-frame sensor will be identical to the full image from the smaller sensor (including DOF) - which is obviously exactly as you'd expect. It shouldn't be interpreted as "the DOF of the uncropped full-frame image will be the same as the DOF of the cropped image" (because it isn't).
Ok here we go again...

When you use a ff sensor, or crop sensor, for any shot, taken with the same lens, from the same point with the same level of magnification during printing/displaying the image, the common portion of the two images ARE IDENTICAL.

There are two things that people forget with respect to comparing FF and cropped sensors.


First, when using a cropped sensor, because you enlarge the image more to (for example print to 8"x10") this you would with a full frame sensor you are enlarging the image more. This has the effect of reducing dof because dof is defined traditionally as acceptable sharp when viewing an 8x10 print. This is why an aps-c camera has a circle of confusion that is smaller (at the sensor) than full frame. It is all relative to aa pure point of light appearing as less than 0.01" on an 8x10 print. That is the origin of the definition of DOF.

But......

This is counter intuitive to what every one claims where ff cameras have lower dof than crop sensors, so let's go to the second point....

With a subject at a set distance, to get the same image height in the view finder, with a crop sensor as full frame, you have two options, either put on a shorter focal length lens, which has more DOF, or move back, which, as you do, for any aperture DOF increases the further away you are.

when you go to a shorter focal length lens the impact on DOF is more than the loss of DOF from the different enlargement ratio so it appears that you get more DOF from a crop sensor than full frame when shooting from the same point ad compensating with changing focal length.

If you keep focal length the same, and move back to compensate for the crop sensor, the increase in DOF as you move back to keep the image on a crop sensor the same in proportion to the sensor, as the ff camera, the increased DOF of the lens is largely offset by the loss of DOF from the enlargement process, and you get relatively the same overall DOF

Set up some examples and play with your DOF calculator, it will surprise you
11-21-2013, 07:38 AM   #14
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Hmm, a DoF argument/discussion begins with everyone being right but coming from different points of view and not clearly laying out all their assumptions. That's never happened before.

Any version of "FF has less/more/same DoF than APS-C" is vague enough to be totally meaningless without additional qualifiers. Adam's 3 point quote sorts it out the main interesting cases, and confusion would probably be reduced if people would say which one they're considering from the outset.
11-21-2013, 09:17 AM   #15
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QuoteOriginally posted by BrianR Quote
Any version of "FF has less/more/same DoF than APS-C" is vague enough to be totally meaningless without additional qualifiers. Adam's 3 point quote sorts it out the main interesting cases, and confusion would probably be reduced if people would say which one they're considering from the outset.
So true and part of the reason why discussion of cross-format DOF is best left alone unless the original inquiry requests that information. It is almost impossible to give a concise answer that makes sense and the online calculators and phone apps only complicate the matter. I suppose somebody at some time should create a sticky with actual comparison images. Until then, it is all anecdotal based on the experience of those who actually shoot multiple formats with the same focal length lenses and the conjecture of those that don't.


Steve
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