Photoclubalpha

I am so confused, to use an old GoonShow phrase.

When I use an APS-C lens on a fullframe camera, the actual functioning focal length increases 1 1/2x. Yes?
So if I use a FF lens on an APS-C camera, does the functioning focal length reduce by the same amount?

I guess I don't understand the way the lenses are labelled, so I can't be sure as to how these numbers relate.

As I have just moved to Sony, have an A7R and an A6000 and a selection of lenses, I am just trying to work out where they fit in the scheme of things.

Thanks for your help and guidance.

I'm not sure if I understood your question correctly, but let me give it a try. First, lenses made for full frame and aps-c use the same focal length labeling. In other words a 50mm lens designed for FF has the same focal length as a 50mm lens designed for aps-c or crop sensors.

As for how the focal length acts on FF versus aps-c, that is where the math comes. Now these aren't exact numbers but they should be close enough.

A 50mm lens on an aps-c camera acts like a 33.5mm lens on a FF.
Multiple by 2/3. (or .667)

A 50mm lens on a FF acts like a 75mm on aps-c.
Multiply by 1.5.

Does this answer your question? There are other differences like depth of field that change when going between FF and aps-c with the same lens. Also, when using a lens specifically designed for aps-c on a FF camera, you will get some black around the edges because the lens was not designed to project onto that size of sensor.

Anyone else feel free to correct me if I'm wrong. I am by no means an expert.

Thank you. It is as I thought but I wasn't confident enough to assume it. Now I know.

Thanks again.

Hi
I read this thread with interest. I have a question, I use a couple of KM mount lenses, 17-35 F/2.8 and 24-85 F/3.5 on my A-57. The lenses were for FF and the camera is APS-C. As I see it only about the centre 2/3rd of the lens diameter is seen by the sensor which is usually where the better performance is. The lenses are not having their edge performance taken into account by the sensor. Hope this makes sense, any comments would be useful.
Cheers
Pete

Here's a diagram I found that shows approximately how much of a FF lens is used with aps-c sensor versus FF sensor. This is why vignetting is not an issue when FF lenses are paired with aps-c sensors.

Thanks, its quite a small centre section used with a FF lens on a APS-C sensor, which may explain why I'm impressed with the centre and edge performance of the 2 lenses mentioned on my A-57.

But it's not quite so simple.....
If I mount my aps-c 18-200 lens on my A7R, for sure, the image inside the vignetting is of a wider angle view than when it is mounted on the A6000. But I would crop off the vignetting, and the image so produced is square and OF approximately THE SAME effective focal length as when on the A6000.
The other way round, a FF lens on the A6000 definitely produces a bigger field of view
.
Given that the vignetting on zoom lenses is variable, depending upon the zoom chosen ( and the lens!) it is really worthwhile, if you have the lenses, to get used to what they will produce, so that the best choice from your 'weaponry' can quickly be made. I have 2 aps-c zooms, which are significantly lighter, cheaper and smaller than the FF equivalents, and using them on the 'wrong' camera is quite convenient!
Of course, prime lenses don't offer this flexibility and an aps-c lens used on a FF camera is not likley to produce added-value unless you need, for example, a wider aperture.

Looking at image quality doesn't seem to reveal any disadvantages either.

Thanks for that explanation Artharis

I often read review sites to get info on cameras and lenses. DPReview being one site.
I have noticed they have consistently given Sony poor marks for Jpeg quality right across the range but I have noticed one exception, the NEX-5N which they actually broke a trend and said Jpeg out put was very good. Looking at Amateur Photographer site, they positively hate Sony but Pentax can't do wrong in their eyes, nor can Canikon. These trends tend to vary across all review sites, so my advice is tread carefully and use user reports more than review sites.

Pete

I also read DPReview, and have often wondered why they go on so much about jpg quality out of camera - surely every serious photographer produces them in his computer, if he needs them at all.
The only justification for producing them in camera that I can see is to send them off speedily by 'phone to an attendant audience, something in which I, for one, have no interest at all.

Your probably right, but I guess the majority of camera users [happy snappers] whether a DSLR or a compact will shoot Jpeg. The more enthusiastic amongst us will as you say start with RAW.
I use both modes but when I do shoot Jpeg I do expect to get a quality picture that can still be edited if necessary.