Showing my ignorance

Can I just ask a silly question just to get things straight in my head.

An 18mm APSC is equivilent to a 24mm FF lens due to the crop factor however if I mount a 24mm FF lens on my K5iis will it give me the same view as a 18mm or 24mm APSC lens?

I keep over thinking it and cant quite grasp the answer...



Yes I know, think arn't I

Not a silly question at all.

24mm. The difference is that the full frame lens provides a larger image circle. The 24mm APSC lens on ff would vignette heavily.

I think. Cue many pages of talk about angle of view and depthe of field that I won't understand.

Actually a 16mm lens on crop is closer to 24mm would look on ff. It makes no difference to angle of view whether the lens was designed for crop or full frame.

A 24mm lens on crop gives an angle of view similar to 35mm on full frame.

The APS-C sensor of the K-5 crops a smaller rectangle (23.7x15.7mm) from the centre of the full frame rectangle (36x24mm), giving a similar view to a lens of about 1.5x the focal length, i.e. approx. a 36mm lens.

Philip

I thought the question was does a 24mm full frame lens on APSC give the same result as a 24mm APSC lens on APSC. That's why I answered 24mm. I'm ready to be baffled now.

Apologies Doctor - you are right. It's an example of understanding the answer but misunderstanding the question!

Philip

Put any 24mm lens on an APS-C Pentax then the 1.5x crop factor will make the field of view the equivalent of a 36mm lens on full frame. The lens doesn't actually change focal length because it has been put on a different camera body.
A lens designed for an APS-C sensor may not cover the whole of a full frame sensor but its focal length remains what it says on the lens, it'll just get a wider view on the larger sensor! What is likely to happen is that the extra field of view onto the larger sensor will give rise to vignetting, but some lenses may suffer more than others.

Youtube comparison: link

Regardless of the size of lens,on APS-C you will get 2/3 of a full frame image.

Jack.

The answer is 42! Now what's the question.......


Thanks for all the responses. My reason for asking is this;

I am interested in taking some star shots and so I was interested in a Wide angle fast lens. I have the 12-24 f4 and the 16-50 f2.8.

I noticed some legacy glass going quite cheap on ebay, as I would be focussing manually anyway I wondered if I could get a bargain.

Obviously not.

You have two very fine lenses

It takes a lot to beat them!
Legacy glass is interesting, and I've tried lots, but I've whittled it down to those two + DA*50-135. Everything else is disappointing (IMO).

The 12-24 would be good place to start, at f/8 on long exposure, ISO 100. Manual mode and experiment

Matt

"The 12-24 would be good place to start, at f/8 on long exposure, ISO 100. Manual mode and experiment"

this will be fine if you intend to do star trails , or a series of 30 sec exposures which i prefer as it cuts down on extraneous light from city/town lights. you blend them together in Startrails.exe or StarTrax both of which are free and very good. another tip i found if you would like a dark blue sky is to use tungsten WB.
if you are tinking of astro photography then probably best finding people that do that for advice as that is a totally different ball game




this is 107 30sec exposures using DA15 @f5.6 put together in Startrails.exe then a bit of tweaking in ps

Thanks guys. I knew that I had pretty good lenses but I was trying to justify buying something to play with Guess I'll have to think of another reason to spend some money I don't have

I have been doing my homework and, once we manage to get a cloudless sky in Norfolk, I shall be out trying my best.

I use a mac so Startrails.exe is out for me. Doing some googling I have found StarStax but I'm sure someone will be along shortly to recommend a mac offering???