[xephias]

Hi everybody,
First of all, I am rather new in photography, so please bear with me.
I took a hundred shots of an owl (I think it was an owl) close to the building I live in.
I set my K20d on a tripod (a $60 tripod), and tried different apertures.
I am not that happy with the pictures that came out.

I noticed several things:
1. I had to correct A LOT for exposure. In the photo below, for example, I had to set EV to -1.5. Is this normal with this lens?
2. When I changed the aperture from 11 to 8 to 5.6 to 2.8, I had to change the exposure compensation each time. As aperture got larger, I needed less EV.
3. I think my photos lack contrast. Do you agree? How can I improve on that?

Any pointers you may give are welcome.
Thanks.

Gaston.

For higher resolution click here.

Attached Images

 

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Last edited by xephias; 01-15-2010 at 10:20 AM. Reason: changed title

[ovim]

Looking at the bigger resolution version it seems that the tree bark is in focus and the bird is sligthly off.

I had a pretty beaten up but working copy of this lens and exposures were pretty much OK at the short end but at the long end it needed EV compensation. I think it has something to do with the fact that the body thinks the lens is at f2.8 when it actually is at f4 and it closes the aperture down too much. If you meter with the green button the exposures are spot on.

[StephenMerola]

Does your lens have the "A" setting? If not, you need to press the green button the set the exposure after each change of aperture. Also, it appears the photo was just focused on the wrong thing.

If your lens does have the "A" setting all you do is put the camera in Aperture priorty mode and set the aperture using the comand wheel.

I have version 2 of this lens and never had any exposure problems.

[StephenMerola]

As far as contrast goes, I think a lot of older lenses have lower contrast. You can adjust this in camera to your liking or in post processing. Play around and have some fun. That is what using old lenses is all about....at least for me.

[xephias]

Now that you mention it, I was a little surprised yesterday, because the aperture went all the way to 2.8, when I was shooting at the 210mm end, but didn't pay too much attention to it.
Will certainly try the green button next time. However, that defeats the purpose of having the "A" setting on the lens a little bit, right?
About the focus on the bird, you may be right... I guess I need more training with MF.
Thanks a lot for answering.

Gaston.

[xephias]

Does your lens have the "A" setting?

Yes, it does. However, what ovim said may be true. When I was at the 210mm end, the camera thought the lens had a maximum aperture of 2.8. I had the lens set on the A position all the time.

This is funny, I never had seen this commented anywhere before (about this lens).

[ovim]

This is funny, I never had seen this commented anywhere before (about this lens).

I've seen some talk about this but not much. On newer variable aperture zooms the lens tells the body the correct aperture when zooming but these old ones are not so sophisticated.

[ovim]

I did some digging and thinking (I've had a few beers so I might be utterly wrong) and took a look at Bojidar Dimitrov's Pentax K-Mount Page and especially "Features and Operation of the KA Mount" and it says:

The least-noticeable change is in the movement of the diaphragm actuator. In the original K-mount its displacement is proportional to the diameter of the diaphragm opening. In the KA mount, it is proportional to the area of the diaphragm opening, and thus to the selected f-stop. This leads to an identical stop-displacement between any two consecutive f-stops

So if I understood correctly it shouldn't matter what f-stop the body "thinks" the lens is at. When the body stops the lens down one f-stop should be one f-stop regardless of the wide open aperture value.

...unless there's something wrong with the aperture mechanism on the lens. On Mark Roberts Vivitar 70-210 Series 1 Zoom page is a Pentax specific note regarding this lens:

Note 2: Pentax-specific: The presence of the of the "A" setting is one of the things makes version 3 so desirable today, but Vivitar/Komine only added this feature part way through the production run so not all version 3 lenses have it: Be sure to check before buying.

Could it be that your copy (and the one I had) has the diaphragm actuator mechanism of a plain K-mount version?

[xephias]

I think I need a couple of beers to process all this info

So if I understood correctly it shouldn't matter what f-stop the body "thinks" the lens is at. When the body stops the lens down one f-stop should be one f-stop regardless of the wide open aperture value.

Maybe this is true. However, it contradicts what I described in point 2. As I increased aperture, I needed less and less EV (*). Therefore, the camera was "more wrong" when aperture was small, and got closer to the real aperture when I increased aperture.

(*) Remember that EV was always negative, so the lens was always more open than what the camera thought...

Am I getting this right?

[ovim]

OK, I missed that (beer made me do it ), so your lens has a different problem than mine. You could check if the aperture arm on the lens is bent, that can cause erratic aperture behaviour. You could also move the aperture arm with your finger to see it's moving evenly and freely and to see that the diaphragm opens and closes ok.

[xephias]

Thanks for all the input. I will try that.

[Lowell Goudge]

In reading all the posts, I would like to suggest a different issue.

The OP says that the lens shows F2.8 in the fiewfinder when wide open at all focal lentghs. Can this be confirmed ?

Can the OP advise whether the camera under exposes (in Av mode) at 70mm by about 1 stop and has much closer to correct exposure at 210mm?

I suspect there are 2 thinfs at play here, first of all this is a variable aperture lens, and it appears there is no logic in the lens to modify the reported aperture as the lens zooms. This would normally not matter because the meter in the camera reads light coming through the lens, however, the K10D and K20D have a different type of microprism focusing screen, and this screen introduces a non linear behavior in the metering that is a function of the lens aperture.

It has been discussed for about 2 years on the forum.

I have attached a link to a post that discusses this behavior

https://www.pentaxforums.com/forums/241716-post69.html

Newer lenses that are variable aperture change the reported maximum and minimum aperture as the lens is zoomed. Older lenses do not.

this would explain the OP's exposure concerns.

the focusing as some have pointed out is behind the subject, focusing is difficult without a split image.

[ovim]

Ok, that sounds familiar, I've only had a K100D and now a K200D so I haven't experienced these issues. But now that you mentioned I remember reading about it here.