Simple questions about 35mm lens on dslr...

Hi,
I've recently been looking to get a good long lens - following the other thread "long zoom" with interest. link

I know that an old 35mm lens on our dslr will increase the effective zoom, so 300mm becomes a 480mm etc.. So in my (cheap) mind I'll get a 100-300 35mm zoom and on the K7 have a 160 to 480mm zoom?

What are the downsides? What happens to the effective apperture? I've searched a bit, but have had no luck of finding out the practicalities of this.

Many thanks,
Rick

First of all I would say forget all about so called crop factors since a 300mm lens remains a 300mm lens no matter what camera you put it on. The only thing that changes is the field of view since an APS-C sensor is smaller than a frame of 35mm film.

Aperture does not change. Old lenses may not be autofocus and may not have the necessary electronics to communicate fully with your camera body.

Ah - that helps. And makes more sense. I was wondering how the effective zoom changes so much and could not quite figure it out. Field of view makes sense.

Looking at the old lens models, what are the ones where autofocus "starts" to work on a new body?

Mainly I am referencing the pentax forums DB link

As Ken says it's down to the 35mm image being 'cropped' by the smaller dimensions of APS-C sensor that gives that 'apparent' change in focal length.

An extremely good reference site for all things Pentax (especially lenses) can be found here link

With Pentax lenses the Pentax-F ones were the first autofocus models, aesthetically not to everyone's taste and quite heavy with noisy autofocus but nothing wrong optically as you would expect. The 'A' series lenses work very well although manual focus.

Pesonally, I would avoid the early 'K' and 'M' lenses but others here will no doubt disagree

I dont have any experience of third party lenses.

I think we are all in agreement that Bojidar's website is the fount of all knowledge in these matters. link

Edit: Snap! Great minds think alike, Greytop

For Pentax, it's the F and FA series.

It'll be worth researching each lens you're interested in individually because some old lenses are more worthwhile than others compared to modern equivalents.

A manual-focus, or even manual-exposure, lens can be worthwhile when there is a good quality-to-price ratio. One can save up to several hundred pounds!

To cover the 50-200/300mm F5.6ish range with a zoom, though, you're better off with a modern lens such as a Pentax DA 50-200mm, Pentax 55-300mm or Tamron 70-300mm.

Wow - thanks for the concise and quick answers all. That's all I really need to know - that website (Bojidar's) is a great source of information.

Cheers
Rick

For zooms I would stick to the Pentax-F or later. I do have one M series zoom (80-200mm f4) that I bought cheaply to use with my M series cameras, although having tried it on digital I'll probably repeat the experiment. The optics are perfect but it has some paint loss, so I'd say it has been well used but also well looked after.

The problem with old manual zooms is that they have no way of reporting focal length to the camera for shake reduction. Prime lenses are fine as you simply tell the camera what focal length of lens is fitted when you switch on, Pentax-A lenses will work as normal after this (with aperture ring in "A" position) apart from obviously being manual focus. With a zoom you'd have to switch off and on again then reset the length if you, well, zoomed.

If you fancy playing with older primes then look for the Pentax-A 50mm f1.7, it should cost around £45 on ebay and is a great lens. Simply switch to manual focus, fit the lens, select 50mm from the menu that appears when you switch on and enjoy. These lenses also appeared with some film SLRs which may be a cheaper way to obtain one.

I don't know what kind of price you were thinking of paying, but if you can stretch to it, you won't do much better than the current Pentax 55-300 DA lens.

On the secondhand market, the Tamron 70-300 offers an awful lot of bang for the buck (very sharp but comes with some purple fringing on high contrast edges that you need to process out on occasion). Even new you might be able to find one of these for not much more than £100, and at that price I'm not sure I'd be that interested in looking for an old secondhand zoom from 10-20 years ago.

Finally, another old prime that is worth looking out for is the compact SMCM 135 f3.5. Really good examples should be in the £50 price range, and it's a great lens.