Manual focus zooms on dslr question


fourfivesix

Link Posted 14/07/2013 - 09:19
Hi I've just been given a couple of pentax film cameras and both came with a Pentax A 28-80mm lens. I would like to try one on my dslr but I'm struggling to work out which focal length to input on the camera when it starts up. Can anyone help me with this please?

Thanks in advance 456
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GaryJohn

Link Posted 14/07/2013 - 10:01
Almost all the lenses I use are analogue, and I tend to enter one and a half times the maximum zoom of the lens as effectively the lens you have translates to approximately 42-120mm in digital terms on a DSLR. So in this case I would enter 120. I don't know if I am correct in doing this - but it works for me. Good luck with them.
Gary

doingthebobs

Link Posted 14/07/2013 - 10:03
You need to set the focal length that you are using!

The focal length setting is for calibrating the Shake reduction on the camera.

You could try setting to a mid position and hope it won't make much difference or just turn the shake reduction off and use the camera as normal, you just don't have the advantage of the shake reduction. When the shutter speed is fast enough and the camera is held still the shake reduction will not be needed anyway. It is very useful when the shutter speed gets low but it needs to have an idea of the focal length used but the older lenses don't tell the camera this like the new lenses do.

You should use the actual focal length marked on the lens as this is what the Shake reduction system needs to work accurately. E.G. If the lens is a 28mm, set 28mm.

Hope this helps.
Bob
Last Edited by doingthebobs on 14/07/2013 - 10:07

fourfivesix

Link Posted 14/07/2013 - 10:11
Thank you I will give both ways a try
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GaryJohn

Link Posted 14/07/2013 - 10:20
An interesting difference of opinion [which doesn't help you a great deal]. I had always assumed that the SRS needed to understand the likely degree of magnification that the sensor would need to deal with, and as a zoom will likely capture a range of focal lengths during a single use, that it was therefore sensible to put in the longest focal length that could be used - and due to the smaller sensor on the DSLR the effective length is longer than that actually written on a pre-digital lens.
Does anyone know if there is an 'official' recommendation of what to do somewhere in the depths of the various FAQ's and user guides ?
Gary

jamieallan

Link Posted 14/07/2013 - 10:49
Gary,
I know many people may disagree but I think that your premise that having a cropped sensor with respect to full frame or 35mm film cameras leads to some form of magnification is incorrect. Having a smaller sensor I my opinion simply leads to capturing a cropped image of the scene. I think it is an advertising gimmick to label a 70-300mm lens as 140-600mm equivalent (as say Olympus 4/3rds do on a label on the barrel of the lens). It's still 300mm at the long end irrespective of the crop factor.
Here is a couple of links to good online articles on the matter

http://daryl-hunter.net/crop-factor-myth-misconstrued-misnomers-and-disseminated...
http://www.luminous-landscape.com/tutorials/understanding-series/dslr-mag.shtml
http://www.clarkvision.com/articles/cropfactor/
Jamie

gwing

Link Posted 14/07/2013 - 10:52
GaryJohn wrote:
I had always assumed that the SRS needed to understand the likely degree of magnification that the sensor would need to deal with,

This is absolutely true but the SRS does its sums based on the focal length of the lens and as such it is always the real focal length you need to input and the sensor size doesn't need to be taken into consideration. The camera already knows what size sensor it has

For a manual zoom the correct answer is 'set the SRS FL to the FL you are actually going to use'. For some of us that means to set it at the wide end, for some the long end, and for some of us a compromise somewhere in the middle.

Rgds,
Rob.

DoctorJeff

Link Posted 14/07/2013 - 11:25
jamieallan wrote:
Gary,
I know many people may disagree but I think that your premise that having a cropped sensor with respect to full frame or 35mm film cameras leads to some form of magnification is incorrect. Having a smaller sensor I my opinion simply leads to capturing a cropped image of the scene. I think it is an advertising gimmick to label a 70-300mm lens as 140-600mm equivalent (as say Olympus 4/3rds do on a label on the barrel of the lens). It's still 300mm at the long end irrespective of the crop factor.

I think you should re-read the quote/URLs that you gave, Jamie.
It is no advertising gimmick to label a 70-300mm lens as 140-600mm equivalent. This is a way of stating exactly how it will behave on a digital camera as compared to a 35mm one.
Some of us still use film, and lenses designed for film, and it is useful to know roughly what to expect when we transfer lenses.
Quite apart from anything else, it does tell one what the shake could be like - try hand held shots with a 500mm mirror lens on a 35mm and then with the same lens on a digital. True, the focal length will not change, but the behaviour of the system will.
Think about it.

Geoff
Water can wear away a stone - but it can't cook lunch
X-5
istDS
K2000
P50.
Lenses Digital: 50-200, 18-55 KAF: 28-80.
Lenses KA & K: SMC-KA f2.0, SMC-K f1.4, SMC-K f1.7 Tokina KA 28-70 , SMC Pentax 70-210 F4, Sigma KA 75-300 , Hanimex 500mm Mirror, and the Tamron Adaptall-2 stuff.
and then there's all the M42 kit, and the accessories ...

GaryJohn

Link Posted 14/07/2013 - 11:26
Thanks Jamie and Rob - very helpful. And hopefully this will cover 456's original question !
Jamie - I will have a proper read of those articles... thank you.
I was aware of the concept of cropping resulting from the sensor being smaller than a 35mm film plane, but have misunderstood the concept of 'focal length'. I was equating focal length to simply what was "seen" by the sensor and assuming that the narrower the field of view actually captured by the sensor meant it must, by definition, be a longer focal length. [ie mistaking cropping for magnification]. I shall have to do some re-thinking.
As ever - the skills and knowledge available on the forum is fantastic.
Gary

Pentaxophile

Link Posted 14/07/2013 - 11:47
EFL (equivalent focal length) is sometimes given, to give people an idea of how a lens would look on 35mm. But the true focal length remains what it is, regardless of crop factor, and this should be the focal length stated on the lens barrel not the 'EFL'.
[link=https://500px.com/will_brealey/[/link]

jamieallan

Link Posted 14/07/2013 - 11:53
Geoff,
I used to own the Olympus 70-300mm. Here's an image of it.




The 140-600mm equiv is in blue and the lettering is at least twice the size of the 70-300mm. I always felt that this was nothing other than an advertising gimmick. I've never compared 35mm and cropped sensor performance using the same long lens so I cannot disagree how they handle. However a cut and paste from the luminous landscape link states:

From Ansel Adams in his classic book “The Camera” (New York Graphic Society 1980).

"All lenses of the same focal length give images of the same size at any given subject distance."

This is why the image on a DSLR is not actually magnified in any way by the 1.6x ‘multiplier’.

"All lenses of the same focal length have the same angle of view for any given format."

It therefore follows that if you change the format you must change the angle of view – smaller format, smaller angle of view and vice versa.

Lens Focal Length:

"Light from a subject point at infinity produces parallel rays which are refracted by the lens to focus at a single point. The distance from the lens to this point is the focal length of the lens."

The focal length is fixed and independent of the ‘film’ format.

Jamie
Last Edited by jamieallan on 14/07/2013 - 11:54

fourfivesix

Link Posted 14/07/2013 - 15:15
So I need to change the focal length every time I zoom in or out?
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doingthebobs

Link Posted 14/07/2013 - 17:15
fourfivesix wrote:
So I need to change the focal length every time I zoom in or out?

If you want to use the SRS to it's best advantage, yes you do.

If you turn the SRS off, you don't need to dial in the focal length. A good rule of thumb is for hand held photography is: keep your shutter speed at or above the the same figure as the focal length. E.G. 50th second for a 50mm, 200th second for a 200mm lens. This helps to get your shutter speed fast enough to stop camera shake. You can go slower but you are more likely to get duff pictures due to camera shake.

Of course don't forget to turn the SRS off when using a tripod or fixed support, it can actually cause camera shake!
Bob
Last Edited by doingthebobs on 14/07/2013 - 17:16

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DoctorJeff

Link Posted 14/07/2013 - 18:25
Jamie,
Any lawyer will tell you it is all in the wording.
I do have to agree with you that it looks like a bit of a gimmick on that lens.
Now, just out of arm's reach I have a P50, fitted with a Tamron Adaptall-2 in 17mm F2.5. When I stick that lens on my K20D it behaves like a 24mm (really, it should be 23.5, but who is counting?). If I stick the 18-55 kit lens onto the P50, it vignettes until I have zoomed out to about 24mm equivalent.
The glass does not change, only the behaviour of the system changes.
You see, I am conditioned (long story) to think of lens, body, and photographer as the system.
Think about using extension tubes - they do not change the focal length of the lens, but they do change the size of the image at the picture plane because they have altered the system. True, you can calculate the effect of an extension tube, but who does? The manufacturers do not bother because they do not know what lens the tubes will be used with, or what combination the tubes will be used in.

I did find your quote from Ansel Adams interesting, in that a 1980 publication pre-dates the Sony Mavica by a year, the Nikon SVC by 6 years, and the Kodak KAF-1400 M1 (generally considered as the first usable DSLR) by 8 years. The statement about the image not being magnified is of course correct, but the "1.6 multplier" does give an image at the picture plane that is larger than the sensor. This is not magnification in the true sense of the word - but it sure does act like it.

Geoff
Water can wear away a stone - but it can't cook lunch
X-5
istDS
K2000
P50.
Lenses Digital: 50-200, 18-55 KAF: 28-80.
Lenses KA & K: SMC-KA f2.0, SMC-K f1.4, SMC-K f1.7 Tokina KA 28-70 , SMC Pentax 70-210 F4, Sigma KA 75-300 , Hanimex 500mm Mirror, and the Tamron Adaptall-2 stuff.
and then there's all the M42 kit, and the accessories ...

Pentaxophile

Link Posted 14/07/2013 - 18:55
fourfivesix wrote:
So I need to change the focal length every time I zoom in or out?

Ideally yes. In practise, it's a pain. I dial in the shortest focal length e.g. 75mm on the 75-150mm. Better too little SR than too much, I think (although I have not tested this!)
[link=https://500px.com/will_brealey/[/link]

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