Exposure question

Hi folks, I have a question about exposure.

I've noticed that when using my SMC Takumar 135/2.5 I cannot shoot wide open in bright daylight because the photo will be overexposed. Even on the max shutter speed of 1/4000 and at ISO 200 (this was on my K100Ds), I have to stop down to around f4 or 5.6 to avoid an overexposed photo.

With my SMC Takumar 85/1.8 I can shoot wide open in the same conditions and not overexpose.

How does this happen? Both lenses have the same filter size. How can the 135 be letting in 'more' light than the 85?

Thanks for any explanations!

there might just be a fault in the camera/lens connection.

If you are shooting in Av, change to M, and try all manual settings.

If this doesn't work, then the camera simply doesn't read the aperture setting on the lens. Or the lens is broken?

Just thoughts

Have you tried full manual shooting?

I'm assuming the lens is a manual, non 'A' one.

With the dial in the M position, the camera lets you check exposure by pushing the AE-L button (at least on my K110d it does!)

I shoot a lot like this on my K110d with older lenses, including M42 ones.

try a nd filter

just a thought not experienced to comment on this post sorry

James

Sorry folks I suppose I should have been clearer . Both these lenses are fully manual with no A setting.

I guess just don't understand the physics of it. I will experiment some more when the sun comes out again. What is occurring seems physically impossible to me, so I am still trying to rule out user error!

there is no physics involved.

the lenses are manual, but the camera has automated metering modes or non automated metering modes.

If you put a manual lens on a camera, set to Av mode, the camera will automatically read the aperture value (aperture ring allowed in Custom menu) and give an appropriate exposure.

I have manual lenses, and they work brilliantly this way.

NOW; once you establish that the camera can't automatically meter the lens, you should try (as mentionned before) to put the camera in Manual mode, and set the aperture (on the lens aperture ring) and the time on the camera.

If the results are still overexposed in a normal lighting condition, then the lens is faulty.
If though, you expose for the bright sky, it might simply be that the wide aperture is too bright for the maximum shutter speed of 1/4000.

Different lenses will have different properties and characteristics, it is therefore not necessarily true that if one lens behaves one way, another should as well.

For all my manual lenses I use stop down meatering - Set on 'M', press green button and exposure is set! Works every time for me, some lenses are better at stop down meatering than others, I just apply the right amount of EV compensation.

I have the Pentax 135 F3.5 and It works faultlessly on centre weighted metering.

We'll have to agree to disagree on this one, I think there is quite a lot of physics involved

I don't see how the lens can be faulty. It's fully manual. I can see the iris closing more with each click of the aperture ring. What else is there that could cause this?

This is what I was trying to ask! The two lenses are of the same vintage, and are from the same range from the same manufacturer. Apart from length they look identical. They also have the same filter size. It seemed to make sense that they use similar/same glass.

I understand that the f number is a function of the focal length, but I assumed, wrongly it appears , that a 135mm @ f2.5 would be darker than an 85mm @ f1.8, simply due to the smaller field of view.

Yeah, this was when I noticed the problem. I was trying to apply EV compensation while using the 135/2.5 and I couldn't compensate anymore as I was already at the max shutter speed and lowest ISO for my camera. So I ended up closing the aperture 2 or 3 stops.

I've also got an Pentax M 135/3.5, my very first prime which has a special place in my heart As you say, it worked perfectly with green button metering.

Not so - not with the K10D (and, I believe, the K20D) anyway.

All the "aperture ring allowed in Custom menu" option means is that the shutter will release - it won't, using manual lenses (pre-A) with that option not set.

The camera can't read the aperture from the lens on pre-A lenses, or on A series lenses not set to A. Indeed even with the lens set to A, the camera doesn't read the aperture, it sets it.

Using the aperture ring to set the aperture, all metering is done wide open, unless you use the stop-down metering feature, with which you get the now well-documented exposure issues with pre-A lenses. (Edit: and Stop-down metering is only available in Manual mode).

The restrictions are all listed on p210 of the K10D manual. Sadly the exposure issues with stop-down metering are not described in the manual. They're fairly well known, though - search this or PentaxForums (or DPR I think) for something like "K10D manual lens exposure problems" and you'll get dozens of hits.

I'm guessing that was a typo, I'm sure that Nathan knows that the camera won't read the actual aperture value on a fully manual lens.

Having said that I think Nathan is actually referring to the technique where you set your camera to AV and point it at a target and then as you turn the aperture ring to close down the aperture you can see the camera slowing down the shutter speed to compensate. I guess the camera is constantly metering the incoming light so it knows the lens aperture is getting smaller, even though there is no mechanical or electrical contact between the lens and camera.

The point is, this technique does not work with K10D or later (not sure about the earlier dSLRs).

It meters wide open, regardless of what you set the aperture ring to. It's because of the KAF2 mount - there is no aperture coupling between the lens and the camera that enables the camera to see (or react to) what aperture the lens is set to, until it takes the picture, at which point the stop down lever will simply move as far as it can.

You can turn the aperture ring all you like, but the aperture does not get smaller until you release the shutter. Then the camera stops the lens down to whatever the aperture is set to, and opens the shutter.

Try it if you don't believe me - look at the camera from the front while you turn the ring

_________
Edit: ISTR reading somewhere that M42 lenses are permanently stopped down to whatever aperture you set them to. I've never owned one, and I have no idea whether I'm either accurately remembering what I read, or if I did, if what I remember reading is true. However, if it is so, then they will behave as you say. Perhaps someone could confirm.

Pre-A series lenses and A series lenses (and newer) set to other than A will always make an exposure at maximum aperture (wide open) when the exposure mode on the body is set to Av.

In order to get a reasonably accurate exposure with these lenses you must use manual (M) exposure mode and use the green button to stop the lens down and take a meter reading (stopped down metering).

I say reasonably accurate exposure because there are well documented issues using these lenses which I'll not repeat here.

Not only that, but it operates in Av (still wide open though), even if it's set to P, Sv, Tv, or TAv in these circumstances.

The K10D that is. Is the 20D or -7 any different?

The K20D behaves exactly the same. Can't speak for the K-7. As far as I know, all Pentax DSLR's behave this way, unless the K-7 is different, which I doubt.