Wednesday, August 12, 2015

Testing the Horseman Optical Exposure Computer along the Alapaha River

I recently discovered this meter made for medium and large format view cameras - the Horseman "Optical Exposure Computer." This is a CdS meter made to slip into the camera just like a film holder and meter directly through the lens - a somewhat uncommon thing, but perhaps a helpful accessory for the intrepid large format photographer?

I found the meter on eBay and got it for next to nothing - about 1/20th the original sale price. There are a few different versions, some made for 6x9 style cameras and some for 4x5, with a correspondingly larger metering area. I got the 6x9 version as I can use it on my Linhof 70, and it has an adapter piece to use it on 4x5 cameras. I figured the smaller meter area wouldn't be too big of a deal, but I'll have to be careful of what exactly is in the meter area!

In use, it was very simple to operate. The biggest issue is the batteries - it was designed for older mercury cells that provide 1.5v, while modern equivalents only hit about 1.35v when fresh. This causes a discrepancy in the readings - but from what I've read it's equivalent to about 1-stop overexposure. This means you can set the ISO to one stop higher than your personal EI. Practically speaking, I metered a gray card at home and compared it to my spot meter, and this seemed to be the correct offset. Testing this for yourself is important.

The results were good. I compared the meter reading to my spot meter calculations before shooting and was pleasantly surprised to see they matched for the most part. When using an R25 filter, I gave it an additional stop of exposure compared to the meter reading as I normally do for any TTL meter (due to the spectral sensitivity of the film). I believe this is a general rule of thumb for red filters.

Here is a video showing the operation of the meter and several images taken on this outing:

Here are the images themselves:

As I mentioned in the video, it wasn't perfect - I underexposed the last image here of the overpass. I'm not sure why though, I might've knocked the ISO setting (which is easy to do on this unit) or perhaps it simply biased to the sky/clouds too much. However, overall I am very pleased to have this meter, especially for close-up images where bellows extension factors in.

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