Understanding 10D Onboard Flash

[jkelley]

Hello,
I am a new 10D owner (about 2 weeks now) having stepped up from a G2. I also own a Elan IIe, but definitely consider myself a novice. I am finding that the immediate feedback I get by using a digital SLR to review pictures right after I take them is proving to be extremely helpful in improving my picture taking success.

I am hoping that someone can give me some insight into the inner working of the onboard flash.Specifically, I was experimenting last night taking a series of indoor shots (6 to 8 feet away) using the flash with the camera set to manual. Shutter speed 1/90, with a range of F stop. ISO was 400. I wanted to stop down the lens (28-105, 3.5-4.5) to F22 to maximize depth of field, but ended up with a very dark exposure. In my particular situation, I found the best I could do was f8 to get a good exposure.

Can someone point me in the right direction here. I could not find anything in the users manual that indicates the smallest aperature that can be used with the built in flash. I believe 1/90 shutter speed falls within the allowed synch speed range. Would a speedlite 550EX solve my problem?

Thanks for any advice. This forum is excellent...

[robertwgross]

In manual mode, the meter is still working, and it will still center on what it thinks is correct exposure for a given scene. However, when you start turning the dial to change aperture all the off to f22, then you are taking responsibility for exposure correctness away from the camera. Since the shutter speed was fixed, and then you stopped down the aperture greatly, I could imagine that the shot was way underexposed.

May I suggest that you begin with one of the more automatic modes? Start with the "Green Box" and see what the camera selects for exposure settings. Then go to Tv or Av and see what it selects. When in Tv mode, try dialing around different speeds, or in Av, try dialing around different apertures. Then try offsetting your exposure. In other words, dial up or down from what the camera wants to do. Right down notes of what you are doing.

Remember, you are running an experiment, and with digital there is virtually no cost!

Finally, once you understand better what is going on, you can try the manual mode with your own settings. Personally, I find that there is very little that I can do with flash that is much better than what the camera will do automatically. Of course, I use a 550EX for serious flash work, and it has some intelligence also.

---Bob Gross---

[jkelley]

Bob,

Thanks for your reply. I guess my question really is why wasn't the flash able to increase its output to account for the stopped down lens. In other words, I hit some limitation with the flash when I stopped down past f8. I'm trying to understand what the limitations of the flash are.

I have done many of the experiments that you suggested in your response. When shooting with the flash, I normally shoot in Tv mode. What I find is that the camera normally selects the largest aperatutre available with the lens (in my case 3.5-4.5) in low light situations. The problem is that doesn't always provide the depth of field that I am looking for.

Can anyone recommend a book/article on the subject (with info specific to the workings of the canon flash). Thanks for any advice...

[PacAce]

jkelley wrote:
Bob,

Thanks for your reply. I guess my question really is why wasn't the flash able to increase its output to account for the stopped down lens. In other words, I hit some limitation with the flash when I stopped down past f8. I'm trying to understand what the limitations of the flash are.

I have done many of the experiments that you suggested in your response. When shooting with the flash, I normally shoot in Tv mode. What I find is that the camera normally selects the largest aperatutre available with the lens (in my case 3.5-4.5) in low light situations. The problem is that doesn't always provide the depth of field that I am looking for.

Can anyone recommend a book/article on the subject (with info specific to the workings of the canon flash). Thanks for any advice...

The 10D manual states that the internal flash has a guide number of 43 at ISO 100. That means, if I'm doing my math correctly, that at an ave. distance of 7 ft., your aperture needs to be set to about 5.6 to get the proper exposure at ISO 100. Since you set your ISO to 400, you gain 2 stops and hence have to set your aperture to 11 for the correct exposure. So, your setting the aperture to f/22 underexposed your shot by 2 stops.

I'm sure using the 550EX would fix this problem.

[robertwgross]

The stupid little built-in flash on a 10D is not very powerful, and it cannot simply increase its output by several stops to offset the whims of the operator. The camera's normal method to deal with this is to vary the aperture to get the exposure correct. But then, if you have switched it out of any automatic mode into manual mode, then you have taken responsibility away from the camera. What is the guide number, just about Zero?

I have a D60, which is very similar, and I only use the little built-in flash for a little fill when in shadows during the day. When I need to do a proper flash job, then I bolt on the 550EX. Now, it has the horsepower to light up the situation by several stops.

Let's see. What do we know? If it were a completely dark situation, then the shutter speed makes little effect on the exposure, as long as it is within certain sync limits. Then in that case, the aperture setting is the only variable that is going to have any great effect. However, if you have a bright situation with ambient light, both shutter and aperture can work together to provide a correct exposure. In your situation, if it is half-dark, then you are halfway between the first situation (aperture only works) and the latter situation (both aperture and shutter work).

---Bob Gross---

[jkelley]

PacAce,

Thanks for your explanation. After my last post I pulled out my Magic Lantern Guide (for my Elan IIe) and my Kodak Pro Photoguide and did some reading. I now understand the calculations and the resulting limitation of the flash based on the guide number.

I agree with your assessment that the 550EX would solve my problem. Looks like I'll need to check under the cushions to see if I can find the $$$.

Thanks for your explanation and all the other responses to my original post

[robertwgross]

Although a 550EX is one solution, it is not the only solution. I happen to own one, plus the ST-2E transmitter. Now I am heading into a job where I need a second flash, so I am likely to go after a 420EX for that. However, there are many situations that I could work with the 420EX in place of a 550EX.

---Bob Gross---