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Whats my focal length!

PENTAX-PLANET
Posted 21/06/2009 - 01:47 Link
When talking about focal length in "35mm terms" could someone explain this to me.

I've recently bought a second-hand Pentax M 50mm lens, does this mean on my K-m this will give me a focal length of 75mm? When comparing it to my kit lens at 50mm the image appears the same. Does it mean that my 18-55mm kit lens gives me a focal length of 27-82.5mm in 35mm terms?

This isn't really an issue but I'd like someone to explain it to me! I assumed that the focal length of a digital lens was different to the focal length of an old 35mm lens but im not sure now!
Neil Richardson

Pentax K-5, 18-55mm DA WR
leytono
Posted 21/06/2009 - 05:56 Link
As I understand it there is a 1.5x multiplying factor with the APS-C sensor which makes your figures right
The focal length of the lens cannot change, but gives the impression of the longer length
So both of the lenses you mentioned act in the same way when used on your K-m

If you are asked to input the focal length on your camera when using the older lenses you use for example in the case of your M 50mm exactly what it says on the tin ie 50mm

Cheers
Rich
Edited by leytono: 21/06/2009 - 06:04
pschlute
Posted 21/06/2009 - 07:27 Link
Rich is correct.

The focal length of a lens relates to the lens, not what camera system you use it on. So a 50 mm lens will always be a 50mm lens.

You will get a different "field of view" if you mount it on a 35 mm camera or a full frame digital camera.
Anvh
Posted 21/06/2009 - 09:21 Link
Yeps only the angle of view (AOV) change but also some other things change as well since the circle of confusion (COC) is also smaller.

COC is how big a spot can be before it becomes unsharp.
With 35mm that is 0.029 mm with an APS-C sensor that is 0.018 mm.
That means that the dof becomes smaller since a circle of 0.021mm appears blurred on APS-C but sharp on 35mm. Since the angle of view change also with an APS-C sensor with a factor of 1.5x you wont really notice it since all your lenses will be used like longer lens anyway.

Simply remembers that the Angle/field of view changes with a factor of 1.5x so that the lens appears 1.5x longer, that's all you need to know.


ps. I like to use the term angle of view above field of view since angle of view relates only to the lens and camera where by the field of view also need a focus distance so that you know where and how large the field is.
Stefan
Comment Image

K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ
Edited by Anvh: 21/06/2009 - 09:23
PENTAX-PLANET
Posted 21/06/2009 - 11:04 Link
Great thanks for the help - everything makes much more sense now

I'm not sure if a full frame sensor would suit my needs - there were a couple of shots I took yesterday where it would have been usefull having a wider angle or view, but I don't think I would want to be without the camera I have if I did have a full frame slr.

Have Pentax got a full frame DSLR planned?
Neil Richardson

Pentax K-5, 18-55mm DA WR
hefty1
Posted 21/06/2009 - 11:15 Link
There's no "full frame" DSLR planned officially (but you never know what the future may bring), however, eBay is awash with older "full frame" Pentax cameras that just happen to use film - try one, you might like it!
Joining the Q
PENTAX-PLANET
Posted 21/06/2009 - 12:12 Link
hefty1 wrote:
There's no "full frame" DSLR planned officially (but you never know what the future may bring), however, eBay is awash with older "full frame" Pentax cameras that just happen to use film - try one, you might like it!

I've not had any experience with a Film SLR - i'd like to try it though, but what pentax film slr would you recommend?
Neil Richardson

Pentax K-5, 18-55mm DA WR
Pentaxophile
Posted 21/06/2009 - 12:53 Link
This thread is a great place to start: link

ME Super seems to be well regarded, very nicely built (all metal) and a very cheap option... but there are many options, all of which are pretty good! I had a P30 - all manual, bit basic, but solid - and you can pick one up for a tenner!
[link=https://500px.com/will_brealey/[/link]
hefty1
Posted 21/06/2009 - 15:02 Link
I personally love my MX - a superb film camera but it's also all-manual which could be a little intimidating if you're new to photography in general.

If you'd like a film camera that will allow you to do things "automatically" while your confidence builds then I'd look at something more recent like the MZ-3, MZ-5, MZ-6 or MZ-7. The latter two (6 + 7) sacrifice a few "pro" features but add scene modes for a shooting experience more like the one you get from your K-m.

With film cameras being so cheap these days (except the LX and MZ-S) you could probably buy a dozen for the price of a half-reasonable lens and give them all a run out to see which you prefer!
Joining the Q
PENTAX-PLANET
Posted 22/06/2009 - 08:21 Link
I'm going to have to start looking aroung then - sounds like it could be quite interesting!
Neil Richardson

Pentax K-5, 18-55mm DA WR
PENTAX-PLANET
Posted 23/06/2009 - 23:44 Link
hefty1 wrote:
I personally love my MX - a superb film camera but it's also all-manual which could be a little intimidating if you're new to photography in general.

If you'd like a film camera that will allow you to do things "automatically" while your confidence builds then I'd look at something more recent like the MZ-3, MZ-5, MZ-6 or MZ-7. The latter two (6 + 7) sacrifice a few "pro" features but add scene modes for a shooting experience more like the one you get from your K-m.

With film cameras being so cheap these days (except the LX and MZ-S) you could probably buy a dozen for the price of a half-reasonable lens and give them all a run out to see which you prefer!

What is the MZ-50 like?
Neil Richardson

Pentax K-5, 18-55mm DA WR
hefty1
Posted 24/06/2009 - 01:27 Link
PENTAX-PLANET wrote:
What is the MZ-50 like?

A little more basic and with a plastic lens mount instead of a metal one, it'd do the job though and should be very cheap! As a general rule in the MZ range the models with a single-digit name (MZ-3, 5, 6, 7 and S) are better then those with the double-digit names (MZ-10, 30, 50, 60) - there's also an MZ-M which is a bit of an odd fish as it's a manual camera dressed in an MZ body, probably best to avoid that one for now.

The thing with film cameras is that they're basically a light-tight box with a shutter, hence the quality of your photos will depend entirely on the lens and film that you use. All the differences between the bodies are really limited to quality of finish and level of automation/compatability.

One other thing to bear in mind is that the MZ-30/50/60 all use the same "crippled" K mount that the digital cameras use, hence they also have problems metering with older lenses but unlike digital there's no screen on the back to check your results and adjust. The MZ-3/5/6/7 use a proper K mount with a physical aperture feeler and will therefore make a better job of utilising your old lenses.

In summary, if someone offers you an MZ-50 for a fiver and you fancy a go at film then buy it; otherwise look for one of the higher models as they can be bought relatively cheaply and will give you a much more satisfying user experience.
Joining the Q
PENTAX-PLANET
Posted 24/06/2009 - 08:06 Link
Metering with my old lenses was something that I was worried about - as those lenses I would like to use, and I've read about it not working very well with lenses without 'A' on the aperture ring. I think i'll give that MZ-50 go and have a mess about with it and look out for MZ-3/5/6/7 if I like it

thanks!
Neil Richardson

Pentax K-5, 18-55mm DA WR

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