DOF and lighting question.

bms85 · Apr 6, 2010

I just got a cheap three light strobe setup to practice with. I don't have a meter so for now I'm just taking a lot of practice shots to learn my manual settings.

I am getting an effect where aperture settings of 4-8 will produce a deep shadow that cuts off the entire bottom half of the photo. I can get a full frame at 1/160 or lower ie 1/60 but then the photo is over exposed. I can also get the same effect with faster shutter speeds by increasing my aperature, but then I loose my depth of field. I'm just curious as to what causes this? I thought if I have lighting wrong my image would just be over exposed or under exposed? Any thoughts?

This photo was taken at ISO 100 - 1/320 - f4

aliciaqw · Apr 6, 2010

I think you need some umbrellas...?

bms85 · Apr 6, 2010

Thanks for the quick reply.

This issue has very little to do with the lighting except maybe for brightness. it is being distributed evenly via two soft boxes and an umbrella. I get that effect from changing my camera settings, I can get the effect to go away by changing my settings as well.

I should have chose a better picture as an example the first one makes the top corners look rounded because its catching parts of the soft boxes.

Same settings as original post except for shutter speed which is now set to 1/250 instead of 1/320. then the slower the shutter the lower the black shadow moves until eventually it is off the frame entirely.

Big Mike · Apr 6, 2010

The problem is your shutter speed.

The shutter travels across the sensor in two parts. The first curtain opens the shutter and the 2nd curtain follows it, closing the shutter and ending the exposure.
At faster shutter speed, the 2nd curtain starts moving before the 1st curtain is all the way open. In other words, at faster shutter speeds, the shutter is never full open.

Now, when it comes to strobe/flash...the burst of light is usually much shorter than the exposure time. 1/1000 for example. So if the flash fires when the shutter is not fully open, part of the image will be blocked by the shutter, resulting in part of the image being dark, just like you have here.

This is why cameras have a max sync speed. That is the fastest shutter speed at which they can safely be used with flash. Older cameras were often around 1/60, but new DSLR cameras are more in the negihborhood of 1/200 or 1/250. That being said, sometimes you have to go a bit slower just to be safe. 1/125 is a good starting point.

As for your exposure...shutter speed has no bearing on the exposure from strobes (besides the sync issue mentioned above). Your exposure is controlled by the aperture, the ISO and the light output from the flash (power and distance).

As such, your in-camera meter (all auto modes) are useless when shooting with studio strobes. You need to have the camera in manual mode and just keep the shutter speed below the max sync speed.

Texas Photo · Apr 6, 2010

Everything Big Mike said nails your problem! I'd just like to explain a little deeper since it sounds like this is all new to you. Keep in mind the ISO determines how sensitive to light your digital canvas is, the lower numbers being the least sensitive and higher quality. The aperture/F-Stop controls "how much" light is allowed on your digital canvas. So, if your image is to light, close down or adjust your aperture to a higher number. Keep moving up and shoot test shots until you get your exposure right. There is more to this to consider, like desired depth of field, etc., but for now this should get you up so you can play with your lights. Good Luck!

Kevin v
www.texasphotoworkshops.com

bms85 · Apr 6, 2010

Thanks guys, that was the explanation I was looking for and it makes perfect sense! I am very new to all of this, I'm a web designer by trade. I got sick of clients bringing my work down with poor photographs so I'm teaching myself to take better ones

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