Flash sync speed & a little math...

[nikon_sam]

Let's see if I can explain this right without having to write a book...

I have a Mamiya m645 1000s and I have found that the 1/60 Flash Sync speed is a bit off (or fast)...
When examining the negs I find that they aren't properly exposed throughout the entire image area...
Measuring top to bottom the image should be 41.5mm...what I get is 32mm of well exposed image (when using flash only)
What I'm figuring 32 divided by 41.5 equals .771...so I'm getting just over 3/4 of my image...now, if I did my math right the shutter is 23% faster than the marked shutter speed so when set at 1/60 it's actually firing at 1/73.8...
1/30 would be around 1/36.9...I get a nicely exposed neg w/flash at 1/30...
Figuring that I don't use flash all that much with this camera it's not that big a deal but I also figure that all speeds are a tad fast...
When exposing at 1/1000 I'm shooting somewhere around 1/1230 and that would account for the 9mm of underexposure at the top of the neg...
I figure that the mirror isn't moving fast enough to get out of the way when the shutter fires...now this only happens at 1/1000 and when the shutter is tripped for the first time...if more than one image is exposed at this speed normally the second or third shots will be exposed correctly...also, if I trip the shutter in the "Multi" mode one or two times before actually taking a picture it will come out fine...it's just that first shot...(if shooting at 1/1000 with the mirror locked up all is good)
I rarely shoot this camera at 1/1000 so again not a big deal...

I had this camera serviced last year and I was told that the shutter curtains were tightened up so this might be the cause and I just installed a new lithium battery and it does read at 6.4 volts...

I do get negs that are easy to print so my exposures are pretty good but now knowing this I might adjust the ASA setting to compensate for the shutter being off...(when using the metered prism I have had it set at 320 when shooting 400 film) so I'm close...

Does all this make sense to anyone??? :bang:

 

[payasam]

You seem to have it all worked out. I'll only say that the difference between 320 and 400 is perhaps important just for the most finicky users of slide film.

 

[sevo]

The M645 has an electronically timed shutter, where only very few situations can be fixed by tightening.

 

[Landshark]

The mirror trips the shutter, it's not in the way.
Is the exposure variance at the top or bottom of the negative and are you sure it isn't there at all speeds, is it over or underexposure?
Although the shutter is electronically timed, that doesn't mean the speeds are accurate throughout the range nor does it mean the shutter has to be consistently fast or slow it is possible to be erratic.

 

[Dwig]

...
What I'm figuring 32 divided by 41.5 equals .771...so I'm getting just over 3/4 of my image...now, if I did my math right the shutter is 23% faster than the marked shutter speed so when set at 1/60 it's actually firing at 1/73.8...

Nice math, but its based on a slightly false premise.

The statement that the "maximum X-sync speed is 1/60" only means that it is equal to or faster than 1/60th but slower than 1/125th, the next speed increment on the dial. While the actual maximum X-sync speed the shutter is capable of is probably not near the 1/125th end of the gap, its probably up to 1/3 stop faster than 1/60th. Its extremely unlikely that any manufacturer would fail to build in some safety factor to allow for the shutter running a little fast.

 

[nikon_sam]

The mirror trips the shutter, it's not in the way.
Is the exposure variance at the top or bottom of the negative and are you sure it isn't there at all speeds, is it over or underexposure?
Although the shutter is electronically timed, that doesn't mean the speeds are accurate throughout the range nor does it mean the shutter has to be consistently fast or slow it is possible to be erratic.

Landshark,

I know it's not the mirror causing this problem...The image is underexposed on the bottom portion of the image...
This is only a real problem when using a Flash...and if I set it to 1/30 the image is evenly exposed...
When shooting in daylight with 400 film the negs are very nice and real easy to print...nice blacks, crisp whites and everything in between...