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07-10-2015, 06:51 PM   #1
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Equivalent focal length with crop factor

So I am to understand that when I'm using my tamron 17-50mm 2.8 on my k5 it is equal to a 25-70mm lens now does this affect the aperture also or is it still considered an f2.8 ?

07-10-2015, 07:03 PM - 1 Like   #2
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Still an f2.8, Iguckert, despite what some Equivalence freaks might say.

Aperture is a property of a lens, not a sensor.
07-10-2015, 07:07 PM   #3
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That is what I thought thank you. But the focal length am I right on that? The only reason I ask is because the lens is built for aps-c.

---------- Post added 07-10-15 at 10:09 PM ----------

Or that means it only has to deal with the image circle that comes out the back of the lens.
07-10-2015, 07:13 PM - 1 Like   #4
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QuoteOriginally posted by lguckert79 Quote
But the focal length am I right on that? The only reason I ask is because the lens is built for aps-c.
Focal length is focal length. That also is a lens property and is not affected by the sensor size. What does change is field of view (FOV) for a particular focal length. Your camera's image frame has a FOV at any particular focal length that is 67% of that same focal length on 35mm film.

When a lens is designed for a particular format, say APS-C, it means that the lens has an image circle at the focal plane (sensor/film) of adequate diameter to prevent vignette at all focus distances. That same lens on a larger format will usually vignette.


Steve


Last edited by stevebrot; 07-10-2015 at 07:23 PM.
07-10-2015, 07:17 PM   #5
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Ok thank you.
07-10-2015, 07:17 PM   #6
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The focal length doesn't change, the aperture doesn't change, the field of view changes with sensor size, so when you say 17-50 on APS-c is 24 to 75 on FF, you're talking strictly the angle of view.

QuoteQuote:
A camera's angle of view depends not only on the lens, but also on the sensor. Digital sensors are usually smaller than 35mm film, and this causes the lens to have a narrower angle of view than with 35mm film, by a constant factor for each sensor (called the crop factor). In everyday digital cameras, the crop factor can range from around 1 (professional digital SLRs), to 1.6 (mid-market SLRs), to around 3 to 6 for compact cameras. So a standard 50mm lens for 35mm photography acts like a 50mm standard "film" lens even on a professional digital SLR, but would act closer to an 80mm lens (1.6 x 50mm) on many mid-market DSLRs, and the 40 degree angle of view of a standard 50mm lens on a film camera is equivalent to a 28 - 35mm lens on many digital SLRs.
https://en.wikipedia.org/wiki/Angle_of_view

So just to rehash. Aperture remains the same, Focal length remains the same, angle of view changes.
07-10-2015, 07:28 PM - 2 Likes   #7
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QuoteOriginally posted by lguckert79 Quote
So I am to understand that when I'm using my tamron 17-50mm 2.8 on my k5 it is equal to a 25-70mm lens now does this affect the aperture also or is it still considered an f2.8 ?
The lens is a 17-50mm f2.8. The camera it is mounted on does not affect these parameters, the focal length and aperture are properties of the lens itself.

If you want to talk equivalence, then that is no longer a lens property, it is affected by sensor size. Your 17-50mm f2.8 has a 17-50mm equivalent FOV, and f2.8 equivalent aperture when mounted on a full-frame camera. When mounted on an APS-C sensor, it has a FF equivalence of 26-75mm f/4.2.

What this means is that these setups will give you exactly the same image, assuming sensors of similar generation and design. Same FOV, same DOF, same SNR, same DR and same perspective, if the same scene is shot from the same subject distance:
17mm f2.8, 1/100s, ISO 100
25mm, f4.2, 1/100s, ISO 220

Note that a Tamron 17-50 lens was designed for APS-C sensors, therefore it will vignette very strongly on a FF body.

Last edited by audiobomber; 07-19-2015 at 04:47 AM.
07-10-2015, 07:33 PM   #8
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QuoteOriginally posted by lguckert79 Quote
Ok thank you.
Like Norm said the "focal length" of say a 50mm lens is the same distance between the main element and the film or sensor regardless if its on a full frame or on an APSc so... the magnification is the same! What you read in lens descriptions is something like "the 50mm lens operate like a 75mm lens on the K5 or K3 and people often think that the magnification is like a 75mm but it isn't. The angle of view or field of view is the same as a 75mm lens not the magnification.

07-10-2015, 08:18 PM   #9
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Oh ok this has really cleared up a lot for me thanks guys for all the great info now I am rethinking selling my 17-50 for a 28 -75 tamron thank you so much
07-10-2015, 11:54 PM   #10
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QuoteOriginally posted by lguckert79 Quote
So I am to understand that when I'm using my tamron 17-50mm 2.8 on my k5 it is equal to a 25-70mm lens now does this affect the aperture also or is it still considered an f2.8 ?
Aperture is the hole in your lens through which light travels to you sensor. The maximum diameter of that hole when your 17-50 is at 17mm is 17/2.8= ~6mm. When it is at 50mm it is 50/2.8= ~18mm (f = focal length in f/2.8).

That doesn't change with the sensor you use. And f/2.8 = f/2.8, just like f/5.6 is f/5.6, 2 meters is 2 meters and a tree is a tree. But it doesn't mean a lot. There are compact cameras out there with tiny sensors and f/2.8 lenses, but they can't match the DoF of an f/2.8 lens on an APS-C sensor that provides a similar angle of view (assuming the same focus distance). Likewise, they normally can't match the dynamic range or noise performance of that APS-C sensor.

So the question is then, what is the significance of having a lens with an f/2.8 aperture. To me it's just a number. By itself it doesn't tell me anything about the capabilities it gives me as a photographer.
07-11-2015, 01:20 AM   #11
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QuoteOriginally posted by lguckert79 Quote
So I am to understand that when I'm using my tamron 17-50mm 2.8 on my k5 it is equal to a 25-70mm lens now does this affect the aperture also or is it still considered an f2.8 ?
It's the field of view that changes. The 17-50 on a K5 will give roughly the same field of view as the 28-75 on a full frame.

QuoteOriginally posted by lguckert79 Quote
Oh ok this has really cleared up a lot for me thanks guys for all the great info now I am rethinking selling my 17-50 for a 28 -75 tamron thank you so much
There are two reasons to do so:
  • you don't need the wider end of the 17-50 (nearly always shooting at 28mm or longer or you have another wide-angle option)
  • you're buying it for use on full frame
07-11-2015, 02:35 AM - 1 Like   #12
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QuoteOriginally posted by audiobomber Quote
.

If you want to talk equivalence, then that is no longer a lens property, it is affected by sensor size. Your 17-50mm f2.8 has a 17-50mm equivalent FOV, and f2.8 equivalent aperture when mounted on a full-frame camera. When mounted on an APS-C sensor, it has a FF equivalence of 26-75mm f/4.2.
Incorrect.

The FF shot at f4.2 is now a stop darker, with all the ramifications.

Why do people still buy into this?
07-11-2015, 02:40 AM   #13
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QuoteOriginally posted by clackers Quote
Incorrect.

The FF shot at f4.2 is now a stop darker, with all the ramifications.

Why do people still buy into this?
Clackers, I told you his before: assuming similar sensor technology, you can bump up the ISO on the FF sensor by 1 stop compared to the APS-C sensor and have similar DR/noise. In fact, the ability to go to higher ISOs without so much of the penalties is why some people want to go to fullframe in the first place.

I wonder why you still want to rehash tautologies like f/2.8 = f/2.8, ISO100 = ISO100, 50mm = 50mm. That's like saying a tree is a tree, a tiger is a tiger or a dog is a dog. It's a waste of time.

edit: you may also want to read this: http://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care/4
edit 2: actually, topicstarter may also want to read this article.

Last edited by starbase218; 07-11-2015 at 03:26 AM.
07-11-2015, 04:36 AM   #14
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QuoteOriginally posted by audiobomber Quote
The lens is a 17-50mm f2.8. The camera it is mounted on does not affect these parameters, the focal length and aperture are properties of the lens itself.

If you want to talk equivalence, then that is no longer a lens property, it is affected by sensor size. Your 17-50mm f2.8 has a 17-50mm equivalent FOV, and f2.8 equivalent aperture when mounted on a full-frame camera. When mounted on an APS-C sensor, it has a FF equivalence of 26-75mm f/4.2.

What this means is that these setups will give you exactly the same image, assuming sensors of similar generation and design. Same FOV, same DOF, same SNR, same DR and same perspective, if the same scene is shot from the same subject distance:
17mm f2.8, 1/100s, ISO 100
25mm, f4.2, 1/100s, ISO 300


Note that a Tamron 17-50 lens was designed for APS-C sensors, therefore it will vignette very strongly on a FF body.
QuoteOriginally posted by clackers Quote
Incorrect.

The FF shot at f4.2 is now a stop darker, with all the ramifications.
No, it isn't darker, because the ISO is changed to provide the same brightness. Actually, looking at it again, I went too strong on the ISO, it should be more like 220 for the FF. In practical terms let's say ISO 250, the nearest 1/3 stop.

Last edited by audiobomber; 07-11-2015 at 04:49 AM.
07-11-2015, 04:44 AM - 3 Likes   #15
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QuoteQuote:
you can bump up the ISO on the FF sensor by 1 stop compared to the APS-C sensor and have similar DR/noise.
APS_c------------------- FF

IS0 100...................ISO 200 - DR and noise are equivalent
1/100s....................1/100s - shutter speed is equivalent
ƒ2.8........................ƒ4 - total light used by the sensor to create the image and DoF are equivalent.

These images are roughly equivalent (ignoring the 1/3 stop) . There is simply no FF advantage. What you gain in DR and noise, you lose to maintain depth of field, should you choose to go that route. The fact that many have been deluded into thinking equivalence gives an advantage to one format or another is pretty crazy. What FF does allow you to do is to accept less DoF for better noise/DR performance in the rare instance where less DoF is acceptable, should you wish to make that trade. The fact that many have bought into FF based on a mis-understanding of the science is irrelevant.

FF gives you more total light, and higher ISO if (and only if) you are willing to accept less DoF(and at most ƒ stops, APS-c gives you the same trade off, only shooting wide open is there an FF advantage.) . If DoF remains the same, all other features remain constant . That's equivalence. You have to go to extreme measures, as in the article you posted, where they took some really bad low light images, to demonstrate any difference at all. And even then APS_c and FF were practically identical. Sometimes the articles people post, in support of their point, don't actually support their case, as in this one.

The reason they had to use a low light image to show any difference at all, was because they know the difference at high high iSO is minimal to start with. The difference between 100 and 400 ISO on APS_c is minimal, so no one cares if you're shooting 100 or 200 or even 400 ISO. Both on the test charts with their little graphs etc, there is simply no discernible difference. between APS_c and FF until 800 ISO, so between 100-400 ISO, there is simply a theoretical difference, that doesn't actually affect real world IQ.

Or as I've been saying for years, an FF can make a really bad image, less really bad, but it in no way implies you can't get equivalent images with APS-c 90% of the time if you are shooting high quality images.

I feel really sorry for the people who bought into FF expecting really amazing low light images, or even marginally better images. You will get some better images. But in many cases you'll be lugging around heavier more expensive equipment to get the same image.

Last edited by normhead; 07-11-2015 at 09:49 AM.
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