For various reasons we're doing a reaction time study involving images that runs on unstandardized equipment: patients' personal computers (unless they don't meet certain requirements).
It's not about vision or individual differences, but group-level intraindividual change in reaction times (attention retraining), so it's not that important that participants all have the same experience as long as it's not confounded with the experimental condition.
Most reaction time studies run on standardised equipment, with a standardised distance from the monitor etc.
We can at most ask participants to keep a certain distance to the screen, but we can't ask them to buy new monitors of course.
So, three basic options arise (assuming our images are in a sufficiently high resolution):
Use constant number of pixels.
Positive: We can use the same images for everyone, if we use a medium number of pixels, we can accomodate most resolutions. Also, potentially everyone sees the same amount of information (i.e. no information lost due to down-scaling).
Negative: With high resolutions, images may look pretty small, and with large monitors, participants may usually position themselves further away from the monitor habitually, so some see "less information".
Use constant display size (i.e. 7 cm wide).
Positive: On large and small monitors with different resolutions, images are the same size.
Negative: Apparently the information used to compute actual display size can be unreliable, so we may end up with different display sizes. We'll probably need different images and people with larger monitors and higher resolutions will be able to see more details.
Use constant fraction of display (i.e. 70% percent wide).
Positive: Fraction of display may do the best job of letting people perceive considering that people probably sit further away from bigger monitors.
Negative: Need images in different sizes (dynamically computed with JS). They may see different levels of detail.
These options could be combined (i.e. constant pixels unless that is less than 50% of the monitor) and there's also the option of letting users zoom (will result in somewhat blurry images though).
Is there any research into this matter and if not, which option sounds best perception-wise?