DSLR and SLR lenses

I have a number of KAF lenses for my 35mm SLR. I've been looking into Pentax's DSLR but i've been told that my current lenses will have the focal lenght changed if I use them in a DSLR. e.g. a 30mm F4.5 will come a 45mm F5.6.

Is this true or is the salesman talking s**t?

Haha! Well, technically the focal length cannot change. A 50mm focal length is 50mm. End of story.

But, the angle-of-view is another thing A 50mm will give you the same angle of view as a 75mm would on a 35mm body. This topic has been discussed many times on this forum, but quite simply if you want really wide angle you'll need something like the SMC-DA 16-45mm or the forthcoming SMC-DA 12-24mm.

Conversely, if you want telephoto, then an SMC-FA 80-320mm behaves very well, giving a very telephoto view of around 480mm

Matt

Why on earth didn't you ask the salesman for the reason for the apparent change in focal length, instead of coming on here and casting aspersions in an uncouth manner?

George

Why on earth did you assume that i didn't already asked the salesperson. The explaination was confusing and quite frankly boring. I just thought someone here could've explained it better.

It could be that the salesperson didn't explain it very well, or it could be i've mis-understood, or the salesperson could be ill-informed.

Basically, i use pentax because of the range of lenses I already own. If I switched to digital but can't use my current lenses as they are designed for, then I will consider other brands. That doesn't necesssary mean i'll switch from pentax, it means will consider other brands.

Thanx MattMatic, That's what I thought.

Best thing I suppose is to take my lenses to the shop and try them out on the digital camera.

Yes, this can be a bit of a problem - I love landscapes and using wide-angle lenses but this is where we lose out.
A 28mm lens effectively becomes a 42mm lens, which in 35mm photography wouldn't really be considered wide-angle at all. The effective aperture is also reduced - great for Depth of Field but not so good for low light situations.
A cheap 50mm lens is likely to be pretty good but a cheap 24mm is almost certain to be optically poor, so a really good 'true' wide-angle for a Pentax DSLR is going to cost a pretty penny. Likewise, a full size (35mm) image sensor would be very costly and take the camera out of reach for most people.
Pentax's current range represents a very sensible compromise and users of older 35mm type lenses are catered for and do at least benefit from the image sensor concentrating on the area for the lens' surface that delivers the highest resolution (image quality tends to drop off toward the edges).

We should also bear in mind that the *istD range is even good enough for most professional applications but 8, 12 and even 20 megapixels are really designed for the pros. - and they could certainly afford good wide-angle lenses!

Sorry, this isn't correct. You don't multiply your aperture at all. A 50mm f1.7 is still f1.7, and behaves like it.

Because of the focal length multiplier you may consider that the DoF changes, but don't forget that you have to take into account the sma;;er circle of confusion (which is key to the DoF calculation), and is smaller for the APS sensor. The net result is that the DoF is actually very slightly shallower on the APS sensor (not the other way around!).

Check out my replies here: https://www.pentaxuser.com/forum/viewtopic.php?t=1440&start=30
And this online DoF calc: http://www.dofmaster.com/digital_coc.html#coccalculator and http://www.dofmaster.com/dofjs.html

Matt

Interesting, just when you think you've got your head round something...........

Yes, the DL manual states "compared to a 35mm camera, the value is roughly one aperture setting lower (the focus range becomes narrower)."

I'd assumed that the effect would be similar to using a 1.5x converter

Sorry for being thicker than a village idiot, but here is the explaination of the apparent change in the focal lenght as I understand it now. The area of CCD in a DSLR is smaller than the exposed area on a 35mm film in a SLR, approx. 1.4 times smaller so using a SLR lens in a DSLR will result in the image being cropped. Therefore it appears that a 28mm lens becomes 28*1.4=39.2mm lens although the lens is still a 28mm.

This means that all brands of DSLR are affected by this change!

Simon,
Yes, you're right. The *ist-D/DS/DL have a "multiplier" of 1.5.
All other makes of DSLR have between 1.3 and 1.6x multiplier. The only exception is the Canon 1Ds which has a full frame sensor - and the price to match it

I did a diagram waaay back that you might find useful in visualising it all: http://www.photon.me.uk/matt/cameras/pentax/istd_focal.htm

Hope that helps!
Matt

Simonsteph wrote:

"Why on earth did you assume that i didn't already asked the salesperson. The explaination was confusing and quite frankly boring."

This is, I should have thought, an extremely simple concept. If you are so easily confused and bored, you might find digital photography is not for you (like spelling and grammar).

George

Dear George,

Once again, you’re just assuming certain things have happened. The salesperson did not mention anything about the size of the sensor. What he did mention was something about the actual fitting of the lens to the camera which did not make sense.

If you have anything more to say what is not constructive, please keep it to yourself. I’m sure you’re quite an ace at DSLR. However, I’m not, if fact this is really the first time I’ve touched on the subject, as you should able to tell.

If you can’t communicate in a humane way, then this forum is not for you.

oh! and thanx again to MattMatic for the help

Simon

I hope you've started to get to grips with the info you wanted - it's a minefield at first and not at all easy to absorb everything at once.

When I started photography meany years ago, for a long time it was baffling, and sometimes those who should have known were not very clear when they tried to answer questions!

So, do hang in there, and I really hope you get a lot of pleasure from your new direction in photography. I have certainly found the DSLR to be a superb and very seductive piece of kit!

Thanx for the encouragement. At least I know where to come for support. People seem friendly and willing to discuss problems and topics in this forum (except for one person).