FF lens on crop sensor.

CBA100

CBA100

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Chris
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I've picked up 2 sigma lens (only because I got them for a cheap price) however I didn't realise at the time of purchasing that one was a full frame lens.

I use a crop A77 and I was wondering if there were any issues using a FF lens on a crop sensor (other than the range change).

I currently only have an 18-55mm SAM lens on my A77 so I'd be looking at keeping one of these Sigmas to accompany the Sony 55-300mm that I also have.

Sigma 28-70mm F2.8 DF EX Aspherical (Full-frame)

Sigma 17-70mm F2.8-4.5 DC Macro (APSC)

Would there be any other issues the FF lens and which would be a better lens to replace the current kit lens?
 
Full frame lenses work on crop ....... like normal
 
The 17-70mm Sigma is a crop sensor lens, so will function perfectly on your A77
 
Unless you are in desperate need of constant f2.8 sell the 28-70 and the 18-55 and keep the 17-70
 
CBA100 said:
Am I correct in that the 28-70 on my A77 will actually be 42-105mm??
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The "crop factor" works for field of view and for depth of field but not for exposure. So your 28-70mm needs multiplying by x 1.5 for the equivalent field of view.

Therefore your sums are correct :D
 
CBA100 said:
Am I correct in that the 28-70 on my A77 will actually be 42-105mm??
Click to expand...

The answer is both Yes and No!

The Lens itself will behave exactly the same regardless of being mounted on an A77 or A99 (which is 'Full frame') - it will still 'be' a 28-70.
- the difference is what the camera body then does with the image the lens projects onto the sensor.

In a 'Crop' camera, like the A77, the smaller size of the sensor means that the image is larger than the sensor, so part of it is lost - just as if the full image had been cropped to a smaller central section.

The effect is the same, in field of view terms, as if you had used a lens with a focal length of 42-105 on a full frame camera.

Note: Both FF and APS-C lenses have their focal lengths given in FF terms, so you if you need to either think in FF tersm, of multiply them all by 1.5

IE You 18-55 gives a field of view (FOV) of a 27-77, your 17-70 gives a FOV of a 25-105 and your 55-300 gives a FOV of a 77-450
 
Faldrax said:
The answer is both Yes and No!

The Lens itself will behave exactly the same regardless of being mounted on an A77 or A99 (which is 'Full frame') - it will still 'be' a 28-70.
- the difference is what the camera body then does with the image the lens projects onto the sensor.

In a 'Crop' camera, like the A77, the smaller size of the sensor means that the image is larger than the sensor, so part of it is lost - just as if the full image had been cropped to a smaller central section.

The effect is the same, in field of view terms, as if you had used a lens with a focal length of 42-105 on a full frame camera.

Note: Both FF and APS-C lenses have their focal lengths given in FF terms, so you if you need to either think in FF tersm, of multiply them all by 1.5

IE You 18-55 gives a field of view (FOV) of a 27-77, your 17-70 gives a FOV of a 25-105 and your 55-300 gives a FOV of a 77-450
Click to expand...
Beat me to it (y)
 
Needlessly long and complex IMO. The answer isn't yes and no. It's yes. The op seems to have an understanding of the crop factor so imho there's no need to confuse things with statements like yes and no :D
 
And I would have given an equally profound NO!

28mm is 28mm.

28mm on the crop lens will give exactly the same field of view as 28mm on the FF lens.
 
CBA100 said:
Am I correct in that the 28-70 on my A77 will actually be 42-105mm??
Click to expand...
No

That would be the approx FF equivalent field of view, but the 28 compares to your current 18 as in its longer, if you're suggesting the 28=42 then your 18=27 if that makes sense?
 
woof woof said:
Needlessly long and complex IMO. The answer isn't yes and no. It's yes. The op seems to have an understanding of the crop factor so imho there's no need to confuse things with statements like yes and no :D
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I'm not sure they do, because they were addingthe crop factor to the FF lens but not the crop lens.

We can assume nowt that's not been expressed.
 
Phil V said:
I'm not sure they do, because they were addingthe crop factor to the FF lens but not the crop lens.

We can assume nowt that's not been expressed.
Click to expand...

The op added the crop factor to the FF lens in their second post and asked if their thinking was correct. It is. As it is I think saying yes and no is needlessly confusing.

But this is the internet and in a few posts time it's going to get worse and if we're not very lucky someone is going to start talking about depth of field and you're probably going to be true to form and start deliberately misrepresenting what other people say...

As I said, needlessly complex.
 
woof woof said:
As I said, needlessly complex.
Click to expand...

I agree, as far as the whole 'crop factor' debate can quickly get needlessly complex - BUT what the OP actually stated was

"Am I correct in that the 28-70 on my A77 will actually be 42-105mm??"

to which the answer is NO - the lens does not miraculously change it's optical properties simply because it's put on a body with a different sized sensor.
Hence the need to a slightly longer post to try and make the distinction clear, and to clarify that the same 'crop factor' will apply to APS-C lenses - because I have seen a number of times posts by people who clearly believe it should only be applied to 'FF' lenses.
 
Very simply yes.

Of course it doesn't physically change its optical properties but will give a fov equivalent to 42-105.
 
The only difference between a full frame lens and a crop lens us that the full frame lens produces a larger image circle. With the full frame lens and larger image circle, your crop camera is only using the very middle of the image circle so you will get no vignetting and less barrel and pincushion distortion. That us to say, the ff lens will produce a better image.
 
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