I knew that the viewfinder was going to be dark because it's a fixed aperture f/8 lens. I'd read the reviews complaining about that, but most of the reviews were written by people who have no idea how cameras work. You need a wide aperture to see through the viewfinder, this is why Auto-Indexing was invented.
Using Live View help to see a little better, but this lens is made for use in the brightest light you can get. Since you're putting a crappy plastic lens on your camera anyway just crank up the ISO and live with the noise. This is supposed to be lo-fi, right?
When I first took the lens out of the box I noticed something weird about the aperture hole. It had not just a single hole but numerous tiny holes around it.
I thought this was pretty odd, but when I started shooting some test shots I noticed how terrible the vignette was. It didn't look random at all. There was a pattern due to this aperture hole layout. The whole point about a Holga is unpredictability. This was a very obvious pattern. Since I was using it on my full frame D800 I decided to set the crop mode to 5:4, this is closer to a square pattern as a real Holga would be so I though the vignette would look more natural. Not the case. I still got a distinct floral pattern. I shot the ceiling to give a better idea of what it looks like. When shooting normal images it isn't quite as obvious, but you can still pick it out.
Well, I thought to myself, this is terrible. Looks nothing like Holga vignette. I switched to DX mode, but that just almost eliminated all of the vignette, and made the lens too long (90mm equiv). Since the lens was cheap plastic, I decided to do some minor surgery. I got out my drill bits and set out to widen the aperture to get rid of the extraneous holes. Before you get all crazy and break out the DeWalt drill, I didn't use a drill. I simply started with the smallest bit and gently reamed out the aperture hole until the aperture was one single hole as opposed one central hole with 8 smaller holes surrounding. I started with a 1/16" bit and ended with a 9/64" bit.
This smoothed out aperture resulting in a much more natural appearing vignette, and in my opinion, much better Holga-esque images.