Exposure times and issues with Flash Photomicrography

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

Moderators: rjlittlefield, ChrisR, Chris S., Pau

Microman2
Posts: 22
Joined: Fri Feb 24, 2017 9:02 am

Exposure times and issues with Flash Photomicrography

Post by Microman2 »

Hello, I am having issues with my Flash Photomicrography setup where the images are not exposing fully when the exposure times are decreased (the fraction is getting bigger). This causes a black, linear region within images that increases in area as the exposure times get faster. I am unsure of the cause of this and would like to know any steps to troubleshoot it. My current setup uses a Canon 60D with a flash-sync cord attached to a Yongnuo 580. The camera cannot read the flash unit with this connection so I am unable to change any settings via my camera. The only setting I can change is the flash unit's power and that is done on the unit itself. I would like to know how I can troubleshoot this:
Image Link:https://photos.google.com/share/AF1QipP ... pITXo2Q0FB

In addition to this inquiry, I am also curious about the typical exposure times used to freeze the motion of moving micro-organisms. What exposure times does anybody use?

rjlittlefield
Site Admin
Posts: 23563
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:

Re: Exposure times and issues with Flash Photomicrography

Post by rjlittlefield »

Your problem is caused by setting too short an exposure time on the camera. With most flashes, the camera shutter time must be long enough that the entire sensor is already open when the pulse of light begins, and stays open until the pulse of light has trailed down to nearly nothing. Canon says that for the 60D, this requires a shutter speed of 1/250 second or slower. EXIF data in your image shows that it was shot at 1/400 second, which explains why you have a black band that is fairly narrow. If you had shot at even faster shutter speed, such as 1/1000 second set on the camera, then the black band would be wider because less of the sensor would be exposed during the flash of light.

When using flash, your effective exposure time will be determined by the length of the flash of light. In turn, that is determined by the power setting on the flash unit, with lower power settings giving shorter flashes. The exact relationship is not linear and is seldom published, but it's in the ballpark of 1/500 second multiplied by the power setting. So, at say 1/16 power, you'd be looking at around 1/8000 second effective exposure time. See for example HERE .

Note that some flashes allow a completely different mode of operation, called "FP focal plane" or "high speed sync" mode, that does allow the camera shutter to be set faster than 1/250 second. But as far as I can see HERE, the Yongnuo flashes do not provide this.

--Rik

Pau
Site Admin
Posts: 6053
Joined: Wed Jan 20, 2010 8:57 am
Location: Valencia, Spain

Re: Exposure times and issues with Flash Photomicrography

Post by Pau »

Of course I 100% agree with Rik.
Here you have more info on flash photomicrography: https://www.photomacrography.net/forum/ ... 63#p161563
At the two first links you can see oscilloscope graphs illustrating flash duration and at the following links you can see different kinds of setups. How do you couple yours?
rjlittlefield wrote:
Thu Dec 29, 2022 9:35 pm
Note that some flashes allow a completely different mode of operation, called "FP focal plane" or "high speed sync" mode, that does allow the camera shutter to be set faster than 1/250 second. But as far as I can see HERE, the Yongnuo flashes do not provide this.
--Rik
That high speed sync" mode is useful in general photography when you want to combine the flash with other light sources but in this case the actual exposure time is the one provided by the camera shutter while you will get much shorter exposure if you use the flash at reduced power and this is the best option for avoiding vibration blur and for freezing movements.
Microman2 wrote:
Thu Dec 29, 2022 8:37 pm
My current setup uses a Canon 60D with a flash-sync cord attached to a Yongnuo 580. The camera cannot read the flash unit with this connection so I am unable to change any settings via my camera. The only setting I can change is the flash unit's power and that is done on the unit itself.
You can use an inexpensive TTL flash cord connecting the camera and flashgun shoes like these:
https://www.ebay.com/sch/i.html?_from=R ... _PrefLoc=2
This is what I have and I'm able to control the flash from the computer via EOS Utility
Pau

Post Reply Previous topicNext topic