Do larger prints require different grades?

I have an Omega D2 enlarger with a cold light head. I printed a fairly low-contrast neg on Ilford Multigrade V 8x10 paper at grade 3 with a Rodenstock 90mm f/4.5 lens. A few days later, I made an enlargement of the same neg with the same lens onto 16x20 paper. I had to push the head all the way up to the top of the rails to cover the entire paper. To my surprise, I had to print with a grade 4 filter to achieve similar tonal separation and black levels as the 8x10 print, and even then, the 16x20 print is slightly lower in contrast compared to the 8x10. Is this normal?

Sort of.
A relatively low contrast light source to begin with, plus additional potential flare due to the larger enlargement.
In addition, there are some subjective elements involved. Smaller prints tend to look more contrasty, even when densitometer readings show they are not. To create the same impression with a large print, you do often have to increase contrast.
It also varies from image to image - again a subjective element.
The preponderance of dark or light subject matter influences our perceptions on this.
Try increasing the viewing light level and see how the two versions respond.

Yes, it is normal.

As the enlargement size goes up the exposure time goes up. However, the amount of stray light from the enlarger stays the same. The stray light times the longer exposure time will lower the print contrast. The stray light comes from light leaks around the enlarger and from light reflected from the paper on to the walls and then back on to the paper. Wrapping up the enlarger's light leaks and painting the walls around the enlarger either deep red or black will cut down on the stray light.

And, as Matt has stated, there are subjective factors at work.

1. That's 4 times the size, subjective contrast is a big factor.
2. I have experienced different results from different paper batches. Was the 16x20 older paper by any chance?

A greater degree of enlargement means the light is lens concentrated per the same surface area as before; so there the image will not only need longer exposure or a wider enlarging lens aperture, but the contrast itself might need to be increased somewhat either through a harder grade or longer development time.

MichaelXX2 said:

Omega D2 enlarger with a cold light head. I printed a fairly low-contrast neg

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this doesn't help
cold light heads are notorious ( at least when I use them they are notorious ) for low contrast and the negative being low contrast ... the others said it much better than me ... they used to say for cold light over process your film by a certain %. maybe this will help for your next batch of film ... would be a good experiment at least
have fun!

Huhhh ? Depends on the cold light. Try something like an Aristo V54 blue-green one.

Think of someone shining a flashlight at night. The further away you are from them, the dimmer the light will be. Light is a physical object consisting of osculating electrical and optical wavelengths, and any number of things can scatter, refract or otherwise slow those wavelengths down. Distance will do it for sure. Raising the enlarger's head will cause more light to be lost due to leaking, scattering and refraction.

Pieter12 said:

2. I have experienced different results from different paper batches. Was the 16x20 older paper by any chance?

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Yes, I got two boxes of 16x20 paper from the guy who sold me the enlarger. He didn't know exactly how old the paper was, so it was probably fairly old. I've also read about pretty significant differences between Ilford Multigrade RC IV and V, as far as contrast and black levels go.

Nicholas Lindan said:

Yes, it is normal.

As the enlargement size goes up the exposure time goes up. However, the amount of stray light from the enlarger stays the same. The stray light times the longer exposure time will lower the print contrast. The stray light comes from light leaks around the enlarger and from light reflected from the paper on to the walls and then back on to the paper. Wrapping up the enlarger's light leaks and painting the walls around the enlarger either deep red or black will cut down on the stray light.

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This makes a lot of sense. My bathroom is my part-time darkroom, and there's a mirror reflecting the light right back from the enlarger's worst light leak.

jnantz said:

this doesn't help
cold light heads are notorious ( at least when I use them they are notorious ) for low contrast and the negative being low contrast ... the others said it much better than me ... they used to say for cold light over process your film by a certain %. maybe this will help for your next batch of film ... would be a good experiment at least
have fun!

Click to expand...

It was an FP4 neg I exposed at EI 64 with the intention of developing it in Perceptol, but I ended up developing it in D76 1+1 since that's all I had on hand. I did reduce developing time, but the negs still came out pretty low-contrast. I much prefer low contrast to not having the information on the neg, of course!

Nicholas Lindan said:

Yes, it is normal.

As the enlargement size goes up the exposure time goes up. However, the amount of stray light from the enlarger stays the same. The stray light times the longer exposure time will lower the print contrast. The stray light comes from light leaks around the enlarger and from light reflected from the paper on to the walls and then back on to the paper. Wrapping up the enlarger's light leaks and painting the walls around the enlarger either deep red or black will cut down on the stray light.

And, as Matt has stated, there are subjective factors at work.

Click to expand...

hm...

Temporarily hang something black over that mirror and anything else nearby that is shiny or white.