What happens in the Camera when I alter ASA/ISO settings

[BADGER.BRAD]

Hello all,
Having run in to a problem running 200 ASA film on a dull day It got me thinking what actually happens inside a mechanical Camera when I alter film speed ? I'm guessing some how the ASA dial is geared to the other settings ? A bit deep I know but I like to understand how what ever I'm using works.

Thanks all.

 

[MattKing]

If your camera includes a built in meter, the change to the film speed setting will cause a change to what the meter recommends.
In some cameras with built in meters, the change in the meter will automatically change the shutter speed or aperture settings of the camera.
In other cameras with built in meters (such as match needle cameras), the change in the meter will allow you to manually change the cameras settings to match, without taking your eye away from the viewfinder.
In other cameras with un-coupled built in meters, you use the meter like a hand meter, and then manually set your camera settings according to the meter's recommendation.
And in cameras without a built in meter, it just serves as a reminder about what film you have in the camera.

 

[Platelayer]

The lightmeter in a mechanical camera is selenium or CdS cell type. Either way, it senses the level of incoming light and moves a needle (visible through the viewfinder in some cameras, hidden in others) up and down. When you press the shutter, the needle is securely clamped in position and the aperture, determined by the position of the needle, is read mechanically in the camera. More advanced cameras with aperture-priority or program modes have different mechanical linkages to set the exposure values.

When you change the ISO setting, a mask moves in front of the sensor to limit the incoming light. The 'mask' is a metal sheet, usually connected to the ISO ring, with a series of holes of increasing size. At the slowest ISO (say, 25 on a lot of cameras), a small hole is used to limit the incoming light, making the subject look darker to the camera. The camera will have to increase exposure accordingly. As the ISO is increased (say, to 400, the maximum on many cameras), the widest hole is used making the same subject appear brighter to the camera, therefore less exposure is given to the film.

Which camera were you using? If it's something like an Olympus Trip or a Canonet, which both have quite a fast slowest shutter speed, you might be just on the limit of the highest exposure the camera will give. Manually setting the widest aperture will stop the camera locking while giving approximately correct exposure.

 

[klownshed]

It affects the meter reading.

If your camera is automatic, this will therefore affect the exposure.

ISO forms part of the exposure triangle. Change ISO and you need to also change shutter speed or aperture (.or both) to keep things balanced.

If your film is rated as ISO 200, changing the ISO dial to 100 has the same effect as exposure compensation by over exposing by 1 stop. Which for an aperture priority caMera would be by a slower shutter speed by half.

I.e. If the 'correct' shutter speed for your chosen aperture is 1/500 at ISO 200, changing the ISO to 100 (by a half) would therefore also halve the shutter speed to 1/250 to balance the equation.

Conversely, setting the ISO to 400 would result in an under exposure of 1 stop or double the shutter speed (1/1000) for the example above.

 

[BADGER.BRAD]

Thanks for the replies much appreciated.I have a Number of Cameras (inherited from my late father) A mixture of SLR's and LowFi point and press's All the SLR's have some sort of light meter built in and the Lowfi cameras have the aperture and ASA speed linked so that really you are only able to change the exposure settings. The camera I was using was a Jenaflex and the problem was made worse by the photo I was trying to take and the light conditions, I was trying to take a photo of two trains in Arley station on the Severn Valley railway (A historic railway in the UK). As the light was poor I was having to use a wide aperture and a slow exposure speed which gave me a possible problem with shake and as the two trains were quite long I was hoping to get the full length of them in, the wide aperture causing a problem with this because of depth of field. I must add I have very little experience. I did also take a number of photos with the Cosmic 35m so I will see how they turn out as well.I was thinking off loading my Praktica and Jenaflex with two different speed films to help me out a little in the future, do you think 100 and 400 ASA would be o.k, I'm presently using 200ASA.

Thanks again all.

 

[Platelayer]

From what I've read, the Jenaflex has an electronic shutter, so it'll be doing the ISO/aperture/shutter speed calculation electronically. My avatar picture on this forum was taken at Kidderminster on the Severn Valley Railway; that was a dull day and I was using 400 speed film and 1/30s quite a lot to get sufficient depth of field in some shots. If you change the film speed from 200 to 400, you'll have one more stop to increase the depth of field or shutter speed. There's a trade off between image quality and speed when choosing a film, but for black and white, I find the image quality of most 400 speed film to be very good, so will choose that unless the weather is particularly good.

 

[Sirius Glass]

No filter will make an overcast day look better nor make better contrast and shadows.

 

[BADGER.BRAD]

I do like the Jenaflex and I think it will be the one I stick with along with the Little Cosmic 35m (I used an earlier Cosmic as a child) I just have one more camera to try Which is a Praktica B100. I have a few lenses which fit both the Jenaflex and the Praktica. I'm from Dudley by the way Platelayer.

 

[Platelayer]

The Praktica B100 looks like an electro-mechanical design, with the needle visible in the viewfinder. If the needle wobbles slightly when you half-press the shutter, that'll be a mechanism trapping the needle in order for the camera to read the shutter speed. Otherwise, the shutter speed is read and transferred electronically to the shutter, the needle just being for display. The exposure compensation feature is useful if you have a scene which is brighter or darker than usual and it looks like it's implemented alongside the ISO setting, forming a basic mechanical calculator. Either way, it's another well-built SLR camera and will give you good photographs. If the exposure is a little way out (negatives too thin or too dense), it's likely to be consistent, so just make an adjustment for the next roll.

I'm actually just outside of the West Midlands proper, but only a short train journey from Kidderminster on the main line, so visit every now and then.

 

[BADGER.BRAD]

Here's a couple of the photos I thought were going to be a struggle ! there not too bad after all. I had to upload to Flickr then download them in a smaller size then post them here as for some reason I could not direct link to the photos on flickr.

 

[BADGER.BRAD]

As a comparison here are some from the Cosmic 35m , It does not seem to like photos with a high contrast but does produce some nice photos. In fairness they were took with no light meter and just using the guide symbols on the front. I will take a meter out next time. Do you think they are over exposed (The ASA is pre set against the aperture and goes up in jumps 20/40/80/160/320 so is a bit of a quess trying to get the film speed of 200 ASA, I have set just over 160) or is it just the limit of the camera ?

 

[Platelayer]

These photographs are great. I especially like the ones of 31271 taken from the bridge. The ones from the Cosmic 35 appear to be a little lower in contrast than the ones from the Jenaflex, at a guess I'd say that's due to the lens coatings.

Look to the shadow areas to compare the exposure - there is detail on the mostly black coach end in the photograph from the Cosmic, where it is completely black in the Jenaflex photograph. The colours are looking less saturated on the Cosmic photographs; with colour film, this is sometimes a sign of overexposure, but could be due to the lens coatings. In the end, exposure should be varied for the scene and personal preference, but if you're unsure of the exposure to use with negative film, especially black and white, err on the side of overexposure.

 

[BADGER.BRAD]

Thanks for the reply Platelayer, I would not have thought to look into the shadows to get a comparison, I see exactly what you are getting at ! Next time I take the two cameras out I may try using the light meter on the Jenaflex whilst setting the aperture as the Cosmic is set (f12 or 13 ) to set the exposure. I may well be out using the Weather Symbols as a quide ! Next I'm going down the tow paths to try and snap a few nice boats that way I can take my bike and either the Grandkids or the dogs whilst enjoying a bit of photography.